diff --git a/interp/actime/os_darwin.go b/interp/actime/os_darwin.go
new file mode 100644
index 00000000..02f7e5a6
--- /dev/null
+++ b/interp/actime/os_darwin.go
@@ -0,0 +1,15 @@
+package actime
+
+import (
+	"io/fs"
+	"syscall"
+	"time"
+)
+
+func GetAtime(info fs.FileInfo) time.Time {
+	stat, ok := info.Sys().(*syscall.Stat_t)
+	if !ok {
+		return info.ModTime()
+	}
+	return time.Unix(stat.Atimespec.Sec, stat.Atimespec.Nsec)
+}
diff --git a/interp/actime/os_linux.go b/interp/actime/os_linux.go
new file mode 100644
index 00000000..f2c61556
--- /dev/null
+++ b/interp/actime/os_linux.go
@@ -0,0 +1,16 @@
+package actime
+
+import (
+	"io/fs"
+	"syscall"
+	"time"
+)
+
+func GetAtime(info fs.FileInfo) time.Time {
+	stat, ok := info.Sys().(*syscall.Stat_t)
+
+	if !ok {
+		return info.ModTime()
+	}
+	return time.Unix(stat.Atim.Sec, stat.Atim.Nsec)
+}
diff --git a/interp/actime/os_other.go b/interp/actime/os_other.go
new file mode 100644
index 00000000..83ac7063
--- /dev/null
+++ b/interp/actime/os_other.go
@@ -0,0 +1,10 @@
+package actime
+
+import (
+	"io/fs"
+	"time"
+)
+
+func GetAtime(info fs.FileInfo) time.Time {
+	return info.ModTime()
+}
diff --git a/interp/actime/os_windows.go b/interp/actime/os_windows.go
new file mode 100644
index 00000000..7afb76b8
--- /dev/null
+++ b/interp/actime/os_windows.go
@@ -0,0 +1,15 @@
+package actime
+
+import (
+	"io/fs"
+	"syscall"
+	"time"
+)
+
+func GetAtime(info fs.FileInfo) time.Time {
+	stat, ok := info.Sys().(*syscall.Win32FileAttributeData)
+	if !ok {
+		return info.ModTime()
+	}
+	return time.Unix(0, stat.LastAccessTime.Nanoseconds())
+}
diff --git a/interp/interp_test.go b/interp/interp_test.go
index 8b406231..689e2e3d 100644
--- a/interp/interp_test.go
+++ b/interp/interp_test.go
@@ -1960,9 +1960,20 @@ var runTests = []runTest{
 		"mkdir d; [ -r d ] && echo r; [ -w d ] && echo w; [ -x d ] && echo x",
 		"r\nw\nx\n",
 	},
+	// -N: true if file exists and was modified since last read
 	{
-		"test -N a",
-		"unsupported unary test op: -N\nexit status 1 #IGNORE",
+		">a; cat a; sleep 0.01; echo 'Hello' >> a; test -N a && echo yes",
+		"yes\n",
+	},
+	// -N: false for non-existent file
+	{
+		"test -N nonexistent",
+		"exit status 1",
+	},
+	// -N: false if file was read after last modification
+	{
+		">a; sleep 0.01; cat a; test -N a; echo $?",
+		"1\n",
 	},
 	{
 		"test -? a",
diff --git a/interp/test.go b/interp/test.go
index 23101cb9..d1af1ec4 100644
--- a/interp/test.go
+++ b/interp/test.go
@@ -12,6 +12,7 @@ import (
 	"golang.org/x/term"
 
 	"mvdan.cc/sh/v3/expand"
+	"mvdan.cc/sh/v3/interp/actime"
 	"mvdan.cc/sh/v3/syntax"
 )
 
@@ -166,8 +167,11 @@ func (r *Runner) unTest(ctx context.Context, op syntax.UnTestOperator, x string)
 	case syntax.TsGIDSet:
 		return r.statMode(ctx, x, os.ModeSetgid)
 	case syntax.TsModif:
-		r.errf("unsupported unary test op: %v\n", op)
-		return false
+		info, err := r.stat(ctx, x)
+		if err != nil {
+			return false
+		}
+		return info.ModTime().After(actime.GetAtime(info))
 	case syntax.TsRead:
 		return r.access(ctx, r.absPath(x), access_R_OK) == nil
 	case syntax.TsWrite: