diff --git a/src/ess/livedata/dashboard/plots.py b/src/ess/livedata/dashboard/plots.py
index 03350a41d..eca38a32c 100644
--- a/src/ess/livedata/dashboard/plots.py
+++ b/src/ess/livedata/dashboard/plots.py
@@ -7,6 +7,7 @@
 import weakref
 from collections.abc import Callable
 from dataclasses import dataclass
+from dataclasses import field as dataclass_field
 from typing import Any, ClassVar, cast
 
 import holoviews as hv
@@ -215,6 +216,70 @@ def _compute_time_info(data: dict[str, sc.DataArray]) -> str | None:
         return f'{end_str} (Lag: {lag_s:.1f}s)'
 
 
+@dataclass(frozen=True)
+class CanvasSpec:
+    """Unit metadata extracted from a scipp DataArray for overlay validation.
+
+    Captures coordinate units (keyed by dimension name) and the value unit.
+    Two overlays are compatible when they share the same coordinate dimensions
+    with matching units and matching value units.
+    """
+
+    coord_units: dict[str, sc.Unit] = dataclass_field(default_factory=dict)
+    value_unit: sc.Unit = dataclass_field(default=sc.units.dimensionless)
+
+    @classmethod
+    def from_data_array(cls, da: sc.DataArray) -> CanvasSpec:
+        """Build from a scipp DataArray using its dimension coordinates."""
+        return cls(
+            coord_units={
+                dim: da.coords[dim].unit for dim in da.dims if dim in da.coords
+            },
+            value_unit=da.unit,
+        )
+
+
+def validate_canvas_spec(
+    entries: list[tuple[str, CanvasSpec]],
+) -> str | None:
+    """Check that overlay entries share compatible units.
+
+    Parameters
+    ----------
+    entries:
+        Pairs of (label, units) for each data-carrying element or layer.
+
+    Returns
+    -------
+    :
+        An error message if units are incompatible, or None if compatible.
+    """
+    if len(entries) < 2:
+        return None
+
+    ref_label, ref = entries[0]
+    for label, other in entries[1:]:
+        if ref.coord_units.keys() != other.coord_units.keys():
+            return (
+                f"Cannot overlay '{ref_label}' and '{label}': "
+                f"dimension mismatch "
+                f"({set(ref.coord_units)} vs {set(other.coord_units)})"
+            )
+        for dim, ref_unit in ref.coord_units.items():
+            if ref_unit != other.coord_units[dim]:
+                return (
+                    f"Cannot overlay '{ref_label}' and '{label}': "
+                    f"unit mismatch for coordinate '{dim}' "
+                    f"({ref_unit} vs {other.coord_units[dim]})"
+                )
+        if ref.value_unit != other.value_unit:
+            return (
+                f"Cannot overlay '{ref_label}' and '{label}': "
+                f"value unit mismatch ({ref.value_unit} vs {other.value_unit})"
+            )
+    return None
+
+
 class Plotter:
     """
     Base class for plots that support autoscaling.
@@ -223,6 +288,8 @@ class Plotter:
     This enables efficient polling-based update detection.
     """
 
+    participates_in_overlay_validation: ClassVar[bool] = True
+
     def __init__(
         self,
         *,
@@ -246,6 +313,7 @@ def __init__(
         """
         self._normalize_to_rate = normalize_to_rate
         self._cached_state: Any | None = None
+        self.canvas_spec: CanvasSpec | None = None
         self._presenters: weakref.WeakSet[PresenterBase] = weakref.WeakSet()
         self.autoscaler_kwargs = kwargs
         self.autoscalers: dict[ResultKey, Autoscaler] = {}
@@ -434,6 +502,16 @@ def compute(
         resolver = title_resolver or TitleResolver()
         plots: list[hv.Element] = []
         try:
+            if self.participates_in_overlay_validation and data:
+                entries = [
+                    (key.job_id.source_name, CanvasSpec.from_data_array(da))
+                    for key, da in data.items()
+                ]
+                if self.layout_params.combine_mode == 'overlay':
+                    error = validate_canvas_spec(entries)
+                    if error is not None:
+                        raise ValueError(error)
+                self.canvas_spec = entries[0][1]
             for data_key, da in data.items():
                 label = resolver.get_legend_label(
                     data_key.job_id.source_name, data_key.output_name
diff --git a/src/ess/livedata/dashboard/roi_readback_plots.py b/src/ess/livedata/dashboard/roi_readback_plots.py
index aa0f1dc84..1680abdaf 100644
--- a/src/ess/livedata/dashboard/roi_readback_plots.py
+++ b/src/ess/livedata/dashboard/roi_readback_plots.py
@@ -71,6 +71,8 @@ class RectanglesReadbackPlotter(Plotter):
     rectangles with per-shape colors based on the ROI index.
     """
 
+    participates_in_overlay_validation = False
+
     def __init__(self, params: RectanglesReadbackParams) -> None:
         super().__init__()
         self._params = params
@@ -177,6 +179,8 @@ class PolygonsReadbackPlotter(Plotter):
     polygons with per-shape colors based on the ROI index.
     """
 
+    participates_in_overlay_validation = False
+
     def __init__(self, params: PolygonsReadbackParams) -> None:
         super().__init__()
         self._params = params
diff --git a/src/ess/livedata/dashboard/roi_request_plots.py b/src/ess/livedata/dashboard/roi_request_plots.py
index 28a0881fc..3f9a03902 100644
--- a/src/ess/livedata/dashboard/roi_request_plots.py
+++ b/src/ess/livedata/dashboard/roi_request_plots.py
@@ -533,6 +533,8 @@ class BaseROIRequestPlotter(Plotter, ABC, Generic[ROIType, ParamsType, Converter
     skip logic, publishing) is handled via a closure-based edit callback.
     """
 
+    participates_in_overlay_validation = False
+
     def __init__(
         self,
         params: ParamsType,
diff --git a/src/ess/livedata/dashboard/widgets/plot_grid_tabs.py b/src/ess/livedata/dashboard/widgets/plot_grid_tabs.py
index c18c63d7a..49e3adbb6 100644
--- a/src/ess/livedata/dashboard/widgets/plot_grid_tabs.py
+++ b/src/ess/livedata/dashboard/widgets/plot_grid_tabs.py
@@ -35,6 +35,7 @@
     SubscriptionId,
 )
 from ..plot_params import PlotAspectType, StretchMode
+from ..plots import CanvasSpec, validate_canvas_spec
 from ..save_filename import build_save_filename_from_cell, make_save_filename_hook
 from ..session_layer import SessionLayer
 from ..session_updater import SessionUpdater
@@ -940,6 +941,7 @@ def _get_session_composed_plot(
             Composed plot from session DMaps/elements, or None if none available.
         """
         plots = []
+        overlay_entries: list[tuple[str, CanvasSpec]] = []
         has_layout = False
         for layer in cell.layers:
             layer_id = layer.layer_id
@@ -967,9 +969,23 @@ def _get_session_composed_plot(
                     has_layout = True
                 plots.append(dmap)
 
+                # Collect overlay units for cross-layer validation
+                # (only data-carrying plotters set canvas_spec)
+                if state is not None and state.plotter is not None:
+                    units = state.plotter.canvas_spec
+                    if units is not None:
+                        overlay_entries.append((layer.config.plot_name, units))
+
         if not plots:
             return None
 
+        # Validate cross-layer unit compatibility before composing
+        error = validate_canvas_spec(overlay_entries)
+        if error is not None:
+            return hv.Text(0.5, 0.5, f"Error: {error}").opts(
+                text_align='center', text_baseline='middle'
+            )
+
         result: hv.DynamicMap | hv.Element
         if len(plots) == 1:
             result = plots[0]
diff --git a/tests/dashboard/plots_test.py b/tests/dashboard/plots_test.py
index 9fa9f6fa5..435dbc31e 100644
--- a/tests/dashboard/plots_test.py
+++ b/tests/dashboard/plots_test.py
@@ -2100,3 +2100,135 @@ def test_bars_plotter_normalizes_when_enabled(self, data_key):
         bars = next(iter(result.values()))
         assert isinstance(bars, hv.Bars)
         assert bars.vdims[0].unit == 'counts/s'
+
+
+def _make_result_key(source_name: str) -> ResultKey:
+    return ResultKey(
+        workflow_id=WorkflowId(instrument='test', namespace='ns', name='wf', version=1),
+        job_id=JobId(source_name=source_name, job_number=uuid.uuid4()),
+        output_name='result',
+    )
+
+
+def _make_1d_da(
+    *, dim: str = 'tof', coord_unit: str = 'ms', value_unit: str = 'counts'
+) -> sc.DataArray:
+    coord = sc.arange(dim, 5, dtype='float64', unit=coord_unit)
+    return sc.DataArray(
+        sc.ones(sizes={dim: 5}, unit=value_unit),
+        coords={dim: coord},
+    )
+
+
+def _units(coord_units: dict[str, str], value_unit: str = 'counts') -> plots.CanvasSpec:
+    return plots.CanvasSpec(
+        coord_units={k: sc.Unit(v) for k, v in coord_units.items()},
+        value_unit=sc.Unit(value_unit),
+    )
+
+
+class TestCanvasSpec:
+    """Tests for CanvasSpec construction."""
+
+    def test_from_data_array_1d(self):
+        da = _make_1d_da(dim='tof', coord_unit='ms', value_unit='counts')
+        units = plots.CanvasSpec.from_data_array(da)
+        assert units.coord_units == {'tof': sc.Unit('ms')}
+        assert units.value_unit == sc.Unit('counts')
+
+    def test_from_data_array_2d(self):
+        da = sc.DataArray(
+            sc.ones(sizes={'y': 4, 'x': 5}, unit='counts'),
+            coords={
+                'x': sc.arange('x', 5, dtype='float64', unit='m'),
+                'y': sc.arange('y', 4, dtype='float64', unit='m'),
+            },
+        )
+        units = plots.CanvasSpec.from_data_array(da)
+        assert units.coord_units == {'x': sc.Unit('m'), 'y': sc.Unit('m')}
+        assert units.value_unit == sc.Unit('counts')
+
+
+class TestValidateCanvasSpec:
+    """Tests for validate_canvas_spec (used at both sites)."""
+
+    def test_single_entry_passes(self):
+        assert plots.validate_canvas_spec([('a', _units({'tof': 'ms'}))]) is None
+
+    def test_matching_units_pass(self):
+        entries = [
+            ('a', _units({'tof': 'ms'})),
+            ('b', _units({'tof': 'ms'})),
+        ]
+        assert plots.validate_canvas_spec(entries) is None
+
+    def test_mismatched_coord_units(self):
+        entries = [
+            ('src_a', _units({'tof': 'ms'})),
+            ('src_b', _units({'tof': 'us'})),
+        ]
+        error = plots.validate_canvas_spec(entries)
+        assert error is not None
+        assert 'tof' in error
+        assert 'ms' in error
+
+    def test_mismatched_value_units(self):
+        entries = [
+            ('a', _units({'tof': 'ms'}, value_unit='counts')),
+            ('b', _units({'tof': 'ms'}, value_unit='counts/us')),
+        ]
+        error = plots.validate_canvas_spec(entries)
+        assert error is not None
+        assert 'value unit mismatch' in error
+
+    def test_mismatched_dims(self):
+        entries = [
+            ('a', _units({'tof': 'ms'})),
+            ('b', _units({'wavelength': 'angstrom'})),
+        ]
+        error = plots.validate_canvas_spec(entries)
+        assert error is not None
+        assert 'dimension mismatch' in error
+
+    def test_error_includes_labels(self):
+        entries = [
+            ('detector_1', _units({'tof': 'ms'})),
+            ('detector_2', _units({'tof': 'us'})),
+        ]
+        error = plots.validate_canvas_spec(entries)
+        assert 'detector_1' in error
+        assert 'detector_2' in error
+
+
+class TestPlotterCanvasSpec:
+    """Integration: plotter stores canvas_spec after compute()."""
+
+    def test_line_plotter_stores_units(self):
+        plotter = plots.LinePlotter.from_params(PlotParams1d())
+        da = _make_1d_da(dim='tof', coord_unit='ms', value_unit='counts')
+        plotter.compute({_make_result_key('src'): da})
+        assert plotter.canvas_spec == _units({'tof': 'ms'})
+
+    def test_image_plotter_stores_units(self):
+        plotter = plots.ImagePlotter.from_params(PlotParams2d())
+        da = sc.DataArray(
+            sc.ones(sizes={'y': 4, 'x': 5}, unit='counts'),
+            coords={
+                'x': sc.arange('x', 5, dtype='float64', unit='m'),
+                'y': sc.arange('y', 4, dtype='float64', unit='m'),
+            },
+        )
+        plotter.compute({_make_result_key('src'): da})
+        assert plotter.canvas_spec == _units({'x': 'm', 'y': 'm'})
+
+    def test_compute_with_mismatched_units_shows_error(self):
+        plotter = plots.LinePlotter.from_params(PlotParams1d())
+        data = {
+            _make_result_key('src_a'): _make_1d_da(coord_unit='ms'),
+            _make_result_key('src_b'): _make_1d_da(coord_unit='us'),
+        }
+        plotter.compute(data)
+        result = plotter.get_cached_state()
+        assert isinstance(result, hv.Overlay)
+        (element,) = result.values()
+        assert isinstance(element, hv.Text)
diff --git a/tests/dashboard/roi_readback_plots_test.py b/tests/dashboard/roi_readback_plots_test.py
index edcf72dce..b685f3273 100644
--- a/tests/dashboard/roi_readback_plots_test.py
+++ b/tests/dashboard/roi_readback_plots_test.py
@@ -422,3 +422,29 @@ def test_lines_plotter_compatible_with_regular_1d_data(self):
         )
         compatible = plotter_registry.get_compatible_plotters({'key': data})
         assert 'lines' in compatible
+
+
+class TestROIReadbackOverlayValidation:
+    """ROI readback plotters are exempt from overlay unit validation."""
+
+    def test_rectangles_readback_does_not_set_canvas_spec(self, result_key):
+        roi = RectangleROI(
+            x=Interval(min=0.0, max=1.0, unit='m'),
+            y=Interval(min=0.0, max=1.0, unit='m'),
+        )
+        da = RectangleROI.to_concatenated_data_array({0: roi})
+        plotter = RectanglesReadbackPlotter(RectanglesReadbackParams())
+        plotter.compute({result_key: da})
+        assert plotter.canvas_spec is None
+
+    def test_polygons_readback_does_not_set_canvas_spec(self, result_key):
+        roi = PolygonROI(
+            x=[0.0, 1.0, 1.0, 0.0],
+            y=[0.0, 0.0, 1.0, 1.0],
+            x_unit='mm',
+            y_unit='mm',
+        )
+        da = PolygonROI.to_concatenated_data_array({0: roi})
+        plotter = PolygonsReadbackPlotter(PolygonsReadbackParams())
+        plotter.compute({result_key: da})
+        assert plotter.canvas_spec is None
diff --git a/tests/dashboard/widgets/plot_grid_tabs_layout_test.py b/tests/dashboard/widgets/plot_grid_tabs_layout_test.py
index 5bd7dbfa5..952cf7b06 100644
--- a/tests/dashboard/widgets/plot_grid_tabs_layout_test.py
+++ b/tests/dashboard/widgets/plot_grid_tabs_layout_test.py
@@ -48,6 +48,7 @@ class FakePlotter:
     def __init__(self, cached_state=None):
         self._cached_state = cached_state
         self._presenters: list[FakePresenter] = []
+        self.canvas_spec = None
 
     def get_cached_state(self):
         return self._cached_state