diff --git a/src/video-grid/NewVideoGrid.tsx b/src/video-grid/NewVideoGrid.tsx
index 80f1314..8922f6a 100644
--- a/src/video-grid/NewVideoGrid.tsx
+++ b/src/video-grid/NewVideoGrid.tsx
@@ -45,6 +45,7 @@ import {
   cycleTileSize,
   appendItems,
   tryMoveTile,
+  resize,
 } from "./model";
 import { TileWrapper } from "./TileWrapper";
 
@@ -82,8 +83,11 @@ const useGridState = (
         }),
       };
 
-      // Step 2: Backfill gaps left behind by removed tiles
-      const grid2 = fillGaps(grid1);
+      // Step 2: Resize the grid if necessary and backfill gaps left behind by
+      // removed tiles
+      // Resizing already takes care of backfilling gaps
+      const grid2 =
+        columns !== grid1.columns ? resize(grid1, columns!) : fillGaps(grid1);
 
       // Step 3: Add new tiles to the end of the grid
       const existingItemIds = new Set(
@@ -205,14 +209,10 @@ export const NewVideoGrid: FC<Props> = ({
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [slotGrid, slotGridGeneration, gridBounds]);
 
-  const [columns] = useReactiveState<number | null>(
-    // Since grid resizing isn't implemented yet, pick a column count on mount
-    // and stick to it
-    (prevColumns) =>
-      prevColumns !== undefined && prevColumns !== null
-        ? prevColumns
-        : // The grid bounds might not be known yet
-        gridBounds.width === 0
+  const columns = useMemo(
+    () =>
+      // The grid bounds might not be known yet
+      gridBounds.width === 0
         ? null
         : Math.max(2, Math.floor(gridBounds.width * 0.0045)),
     [gridBounds]
diff --git a/src/video-grid/model.ts b/src/video-grid/model.ts
index 4a6cc32..6ca352e 100644
--- a/src/video-grid/model.ts
+++ b/src/video-grid/model.ts
@@ -418,6 +418,11 @@ export function appendItems(items: TileDescriptor[], g: Grid): Grid {
   };
 }
 
+const largeTileDimensions = (g: Grid): [number, number] => [
+  Math.min(3, Math.max(2, g.columns - 1)),
+  2,
+];
+
 /**
  * Changes the size of a tile, rearranging the grid to make space.
  * @param tileId The ID of the tile to modify.
@@ -433,9 +438,7 @@ export function cycleTileSize(tileId: string, g: Grid): Grid {
 
   // The target dimensions, which toggle between 1×1 and larger than 1×1
   const [toWidth, toHeight] =
-    fromWidth === 1 && fromHeight === 1
-      ? [Math.min(3, Math.max(2, g.columns - 1)), 2]
-      : [1, 1];
+    fromWidth === 1 && fromHeight === 1 ? largeTileDimensions(g) : [1, 1];
 
   // If we're expanding the tile, we want to create enough new rows at the
   // tile's target position such that every new unit of grid area created during
@@ -547,3 +550,46 @@ export function cycleTileSize(tileId: string, g: Grid): Grid {
   // Fill any gaps that remain
   return fillGaps(gappyGrid);
 }
+
+/**
+ * Resizes the grid to a new column width.
+ */
+export function resize(g: Grid, columns: number): Grid {
+  const result: Grid = { columns, cells: [] };
+  const [largeColumns, largeRows] = largeTileDimensions(result);
+
+  // Copy each tile from the old grid to the resized one in the same order
+
+  // The next index in the result grid to copy a tile to
+  let next = 0;
+
+  for (const cell of g.cells) {
+    if (cell?.origin) {
+      const [nextColumns, nextRows] =
+        cell.columns > 1 || cell.rows > 1 ? [largeColumns, largeRows] : [1, 1];
+
+      // If there isn't enough space left on this row, jump to the next row
+      if (columns - column(next, result) < nextColumns)
+        next = columns * (Math.floor(next / columns) + 1);
+      const nextEnd = areaEnd(next, nextColumns, nextRows, result);
+
+      // Expand the cells array as necessary
+      if (result.cells.length <= nextEnd)
+        result.cells.push(...new Array(nextEnd + 1 - result.cells.length));
+
+      // Copy the tile into place
+      forEachCellInArea(next, nextEnd, result, (_c, i) => {
+        result.cells[i] = {
+          item: cell.item,
+          origin: i === next,
+          columns: nextColumns,
+          rows: nextRows,
+        };
+      });
+
+      next = nextEnd + 1;
+    }
+  }
+
+  return fillGaps(result);
+}
diff --git a/test/video-grid/model-test.ts b/test/video-grid/model-test.ts
index 5765915..dd25e8a 100644
--- a/test/video-grid/model-test.ts
+++ b/test/video-grid/model-test.ts
@@ -21,6 +21,7 @@ import {
   fillGaps,
   forEachCellInArea,
   Grid,
+  resize,
   row,
   tryMoveTile,
 } from "../../src/video-grid/model";
@@ -386,3 +387,52 @@ abb
 ccd
 cce`
 );
+
+function testResize(
+  title: string,
+  columns: number,
+  input: string,
+  output: string
+): void {
+  test(`resize ${title}`, () => {
+    expect(showGrid(resize(mkGrid(input), columns))).toBe(output);
+  });
+}
+
+testResize(
+  "contracts the grid",
+  2,
+  `
+abbb
+cbbb
+ddde
+dddf
+gh`,
+  `
+af
+bb
+bb
+ch
+dd
+dd
+eg`
+);
+
+testResize(
+  "expands the grid",
+  4,
+  `
+af
+bb
+bb
+ch
+dd
+dd
+eg`,
+  `
+bbbc
+bbbf
+addd
+hddd
+ge`
+);