diff --git a/slime_plugins/mbridge/__init__.py b/slime_plugins/mbridge/__init__.py
index 9a918fdbdc..b985ef9d3c 100644
--- a/slime_plugins/mbridge/__init__.py
+++ b/slime_plugins/mbridge/__init__.py
@@ -1,4 +1,5 @@
 from .deepseek_v32 import DeepseekV32Bridge
+from .gemma3 import Gemma3BridgeFix
 from .glm4 import GLM4Bridge
 from .glm4moe import GLM4MoEBridge
 from .glm4moe_lite import GLM4MoELiteBridge
@@ -8,6 +9,7 @@
 from .qwen3_next import Qwen3NextBridge
 
 __all__ = [
+    "Gemma3BridgeFix",
     "GLM4Bridge",
     "GLM4MoEBridge",
     "GLM4MoELiteBridge",
diff --git a/slime_plugins/mbridge/gemma3.py b/slime_plugins/mbridge/gemma3.py
new file mode 100644
index 0000000000..81ff7ff451
--- /dev/null
+++ b/slime_plugins/mbridge/gemma3.py
@@ -0,0 +1,22 @@
+import functools
+
+import torch.nn.functional as F
+from mbridge.core import register_model
+from mbridge.models import Gemma3Bridge
+
+
+# Gemma uses GeGLU (GELU with tanh approximation + gated linear unit), not SwiGLU.
+# The upstream mbridge base config hardcodes F.silu which is incorrect for all Gemma models.
+# See: https://developers.googleblog.com/en/gemma-explained-new-in-gemma-2/
+_gelu_tanh = functools.partial(F.gelu, approximate="tanh")
+
+
+@register_model("gemma3")
+class Gemma3BridgeFix(Gemma3Bridge):
+    """Override Gemma3Bridge to fix activation: GeGLU instead of SwiGLU."""
+
+    def _build_config(self):
+        config = super()._build_config()
+        config.activation_func = _gelu_tanh
+        config.bias_activation_fusion = False
+        return config
diff --git a/tests/test_gemma3_bridge_activation.py b/tests/test_gemma3_bridge_activation.py
new file mode 100644
index 0000000000..d69de59b73
--- /dev/null
+++ b/tests/test_gemma3_bridge_activation.py
@@ -0,0 +1,86 @@
+import importlib.util
+import sys
+import types
+from pathlib import Path
+
+import pytest
+import torch
+
+
+def install_bridge_stubs():
+    """Install minimal stubs so the gemma3 plugin can be imported without real dependencies."""
+    megatron_mod = types.ModuleType("megatron")
+    core_mod = types.ModuleType("megatron.core")
+    sys.modules["megatron"] = megatron_mod
+    sys.modules["megatron.core"] = core_mod
+
+    mbridge_mod = types.ModuleType("mbridge")
+    mbridge_core_mod = types.ModuleType("mbridge.core")
+    mbridge_models_mod = types.ModuleType("mbridge.models")
+
+    def register_model(_names):
+        def decorator(cls):
+            return cls
+        return decorator
+
+    class Gemma3Bridge:
+        """Stub that mimics the upstream mbridge Gemma3Bridge."""
+
+        def _build_config(self):
+            # Simulates upstream returning a config with wrong activation
+            return types.SimpleNamespace(
+                activation_func=torch.nn.functional.silu,
+                bias_activation_fusion=True,
+            )
+
+    mbridge_core_mod.register_model = register_model
+    mbridge_models_mod.Gemma3Bridge = Gemma3Bridge
+
+    sys.modules["mbridge"] = mbridge_mod
+    sys.modules["mbridge.core"] = mbridge_core_mod
+    sys.modules["mbridge.models"] = mbridge_models_mod
+
+
+def load_gemma3_module():
+    install_bridge_stubs()
+    module_path = Path(__file__).resolve().parents[1] / "slime_plugins" / "mbridge" / "gemma3.py"
+    module_name = "test_gemma3_bridge_module"
+    sys.modules.pop(module_name, None)
+    spec = importlib.util.spec_from_file_location(module_name, module_path)
+    module = importlib.util.module_from_spec(spec)
+    assert spec.loader is not None
+    spec.loader.exec_module(module)
+    return module
+
+
+@pytest.mark.unit
+def test_gemma3_bridge_uses_gelu_tanh_not_silu():
+    """Gemma models use GeGLU (gelu_tanh + GLU), not SwiGLU (silu + GLU).
+
+    See: https://developers.googleblog.com/en/gemma-explained-new-in-gemma-2/
+    """
+    module = load_gemma3_module()
+    bridge = module.Gemma3BridgeFix.__new__(module.Gemma3BridgeFix)
+    config = bridge._build_config()
+
+    activation = config.activation_func
+
+    # Verify it's gelu with tanh approximation, not silu
+    x = torch.tensor([1.0, -1.0, 0.5])
+    expected = torch.nn.functional.gelu(x, approximate="tanh")
+    actual = activation(x)
+    assert torch.equal(actual, expected), f"Expected gelu_tanh output, got {actual}"
+
+    # Verify it's NOT silu
+    silu_out = torch.nn.functional.silu(x)
+    assert not torch.equal(actual, silu_out), "Activation should not be silu"
+
+
+@pytest.mark.unit
+def test_gemma3_bridge_disables_bias_activation_fusion():
+    """GeGLU has no fused kernel in Megatron, so bias_activation_fusion must be False."""
+    module = load_gemma3_module()
+    bridge = module.Gemma3BridgeFix.__new__(module.Gemma3BridgeFix)
+    config = bridge._build_config()
+
+    assert config.bias_activation_fusion is False