diff --git a/crates/typst-library/src/layout/grid/mod.rs b/crates/typst-library/src/layout/grid/mod.rs
index 6616c3311..369df11ee 100644
--- a/crates/typst-library/src/layout/grid/mod.rs
+++ b/crates/typst-library/src/layout/grid/mod.rs
@@ -755,7 +755,14 @@ impl Show for Packed<GridCell> {
 
 impl Default for Packed<GridCell> {
     fn default() -> Self {
-        Packed::new(GridCell::new(Content::default()))
+        Packed::new(
+            // Explicitly set colspan and rowspan to ensure they won't be
+            // overridden by set rules (default cells are created after
+            // colspans and rowspans are processed in the resolver)
+            GridCell::new(Content::default())
+                .with_colspan(NonZeroUsize::ONE)
+                .with_rowspan(NonZeroUsize::ONE),
+        )
     }
 }
 
diff --git a/crates/typst-library/src/model/table.rs b/crates/typst-library/src/model/table.rs
index 6f4461bd4..373230897 100644
--- a/crates/typst-library/src/model/table.rs
+++ b/crates/typst-library/src/model/table.rs
@@ -770,7 +770,14 @@ impl Show for Packed<TableCell> {
 
 impl Default for Packed<TableCell> {
     fn default() -> Self {
-        Packed::new(TableCell::new(Content::default()))
+        Packed::new(
+            // Explicitly set colspan and rowspan to ensure they won't be
+            // overridden by set rules (default cells are created after
+            // colspans and rowspans are processed in the resolver)
+            TableCell::new(Content::default())
+                .with_colspan(NonZeroUsize::ONE)
+                .with_rowspan(NonZeroUsize::ONE),
+        )
     }
 }
 
diff --git a/tests/ref/issue-6399-grid-cell-colspan-set-rule.png b/tests/ref/issue-6399-grid-cell-colspan-set-rule.png
new file mode 100644
index 000000000..a40eda78d
Binary files /dev/null and b/tests/ref/issue-6399-grid-cell-colspan-set-rule.png differ
diff --git a/tests/ref/issue-6399-grid-cell-rowspan-set-rule.png b/tests/ref/issue-6399-grid-cell-rowspan-set-rule.png
new file mode 100644
index 000000000..a40eda78d
Binary files /dev/null and b/tests/ref/issue-6399-grid-cell-rowspan-set-rule.png differ
diff --git a/tests/suite/layout/grid/colspan.typ b/tests/suite/layout/grid/colspan.typ
index 707a9456b..6e43270c3 100644
--- a/tests/suite/layout/grid/colspan.typ
+++ b/tests/suite/layout/grid/colspan.typ
@@ -140,3 +140,7 @@
   [e], [g], grid.cell(colspan: 2)[eee\ e\ e\ e],
   grid.cell(colspan: 4)[eeee e e e]
 )
+
+--- issue-6399-grid-cell-colspan-set-rule ---
+#set grid.cell(colspan: 2)
+#grid(columns: 3, [hehe])
diff --git a/tests/suite/layout/grid/rowspan.typ b/tests/suite/layout/grid/rowspan.typ
index 88aa34c65..5fc28b9be 100644
--- a/tests/suite/layout/grid/rowspan.typ
+++ b/tests/suite/layout/grid/rowspan.typ
@@ -488,3 +488,7 @@
   table.cell(rowspan: 15, align: horizon, lets-repeat((rotate(-90deg, reflow: true)[*All Tests*]), 3)),
   ..([123], [456], [789]) * 15
 )
+
+--- issue-6399-grid-cell-rowspan-set-rule ---
+#set grid.cell(rowspan: 2)
+#grid(columns: 2, [hehe])