diff --git a/crates/engine/src/cmd/tools.rs b/crates/engine/src/cmd/tools.rs
index b02408dd2..582189473 100644
--- a/crates/engine/src/cmd/tools.rs
+++ b/crates/engine/src/cmd/tools.rs
@@ -629,14 +629,19 @@ pub fn specs() -> Vec<CommandSpec> {
         .params(r#"{"header"?: pt, "footer"?: pt}"#),
         CommandSpec::new("table.cellMargins", "Cell Margins", "Table Layout › Alignment", |s, v| {
             let Some((tp, _, _)) = s.sel.focus.path.cell() else { return Err(CmdError::Disabled("not in a table".into())) };
+            let story = s.sel.focus.story;
+            let current = {
+                let table = s.doc.table(story, &tp).ok_or_else(|| CmdError::Failed("no table".into()))?;
+                let style = table.props.style.as_deref().and_then(|id| s.doc.styles.table_style(id));
+                table.props.cell_margins.or(style.and_then(|st| st.parts.cell_margins)).unwrap_or([0.0, 5.4, 0.0, 5.4])
+            };
             let m = [
-                p::f32(v, "top").unwrap_or(0.0),
-                p::f32(v, "left").unwrap_or(5.4),
-                p::f32(v, "bottom").unwrap_or(0.0),
-                p::f32(v, "right").unwrap_or(5.4),
+                p::f32(v, "top").unwrap_or(current[0]),
+                p::f32(v, "left").unwrap_or(current[1]),
+                p::f32(v, "bottom").unwrap_or(current[2]),
+                p::f32(v, "right").unwrap_or(current[3]),
             ]
             .map(|x| x.clamp(0.0, 144.0));
-            let story = s.sel.focus.story;
             s.doc.table_mut(story, &tp)?.props.cell_margins = Some(m);
             for path in s.doc.para_paths(story).into_iter().filter(|p| p.0.starts_with(&tp.0) && p.0.len() > tp.0.len()) {
                 s.doc.para_mut(story, &path)?.touch();
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index 4031e230a..b3231be92 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -8491,3 +8491,36 @@ fn rtf_list_round_trip_keeps_text() {
         assert_eq!(d.plain_text(StoryRef::Body), "first\nsecond", "{cmd}");
     }
 }
+
+/// #3490: `table.cellMargins` preserves existing margins for any sides not passed.
+#[test]
+fn table_cell_margins_preserves_unpassed_sides() {
+    let mut s = s();
+    run(&mut s, "insert.table", json!({"rows": 1, "cols": 1}));
+    let (tp, _, _) = s.sel.focus.path.cell().unwrap();
+
+    // 1. Updating 'top' only retains default left (5.4), bottom (0.0), and right (5.4).
+    run(&mut s, "table.cellMargins", json!({"top": 20.0}));
+    let margins = s.doc.table(StoryRef::Body, &tp).unwrap().props.cell_margins.unwrap();
+    assert_eq!(margins, [20.0, 5.4, 0.0, 5.4]);
+
+    // 2. Updating 'left' subsequently retains previously set top (20.0), bottom (0.0), and right (5.4).
+    run(&mut s, "table.cellMargins", json!({"left": 10.0}));
+    let margins = s.doc.table(StoryRef::Body, &tp).unwrap().props.cell_margins.unwrap();
+    assert_eq!(margins, [20.0, 10.0, 0.0, 5.4]);
+
+    // 3. Updating 'bottom' retains top (20.0), left (10.0), and right (5.4).
+    run(&mut s, "table.cellMargins", json!({"bottom": 8.0}));
+    let margins = s.doc.table(StoryRef::Body, &tp).unwrap().props.cell_margins.unwrap();
+    assert_eq!(margins, [20.0, 10.0, 8.0, 5.4]);
+
+    // 4. Updating 'right' retains top (20.0), left (10.0), and bottom (8.0).
+    run(&mut s, "table.cellMargins", json!({"right": 15.0}));
+    let margins = s.doc.table(StoryRef::Body, &tp).unwrap().props.cell_margins.unwrap();
+    assert_eq!(margins, [20.0, 10.0, 8.0, 15.0]);
+
+    // 5. Updating multiple sides at once retains unpassed sides.
+    run(&mut s, "table.cellMargins", json!({"top": 0.0, "bottom": 0.0}));
+    let margins = s.doc.table(StoryRef::Body, &tp).unwrap().props.cell_margins.unwrap();
+    assert_eq!(margins, [0.0, 10.0, 0.0, 15.0]);
+}