diff --git a/amaranth/hdl/ast.py b/amaranth/hdl/ast.py
index 0b4939d..b431a6d 100644
--- a/amaranth/hdl/ast.py
+++ b/amaranth/hdl/ast.py
@@ -263,7 +263,7 @@ class Value(metaclass=ABCMeta):
 
     def __abs__(self):
         if self.shape().signed:
-            return Mux(self >= 0, self, -self)
+            return Mux(self >= 0, self, -self)[:len(self)]
         else:
             return self
 
diff --git a/tests/test_hdl_ast.py b/tests/test_hdl_ast.py
index af4b175..5878a6c 100644
--- a/tests/test_hdl_ast.py
+++ b/tests/test_hdl_ast.py
@@ -675,6 +675,17 @@ class OperatorTestCase(FHDLTestCase):
         with self.assertRaises(TypeError):
             hash(Const(0) + Const(0))
 
+    def test_abs(self):
+        s = Signal(4)
+        self.assertRepr(abs(s), """
+        (sig s)
+        """)
+        s = Signal(signed(4))
+        self.assertRepr(abs(s), """
+        (slice (m (>= (sig s) (const 1'd0)) (sig s) (- (sig s))) 0:4)
+        """)
+        self.assertEqual(abs(s).shape(), unsigned(4))
+
 
 class SliceTestCase(FHDLTestCase):
     def test_shape(self):