diff --git a/src/commands/upsert-terrain-command.ts b/src/commands/upsert-terrain-command.ts
new file mode 100644
index 00000000..29b05c39
--- /dev/null
+++ b/src/commands/upsert-terrain-command.ts
@@ -0,0 +1,92 @@
+import { createOpaqueId } from "../core/ids";
+import { cloneEditorSelection, type EditorSelection } from "../core/selection";
+import type { ToolMode } from "../core/tool-mode";
+import {
+  cloneTerrain,
+  getTerrainKindLabel,
+  type Terrain
+} from "../document/terrains";
+
+import type { EditorCommand } from "./command";
+
+interface UpsertTerrainCommandOptions {
+  terrain: Terrain;
+  label?: string;
+}
+
+function setSingleTerrainSelection(terrainId: string): EditorSelection {
+  return {
+    kind: "terrains",
+    ids: [terrainId]
+  };
+}
+
+function createDefaultTerrainCommandLabel(isNewTerrain: boolean): string {
+  const action = isNewTerrain ? "Create" : "Update";
+  return `${action} ${getTerrainKindLabel().toLowerCase()}`;
+}
+
+export function createUpsertTerrainCommand(
+  options: UpsertTerrainCommandOptions
+): EditorCommand {
+  const nextTerrain = cloneTerrain(options.terrain);
+  let previousTerrain: Terrain | null = null;
+  let previousSelection: EditorSelection | null = null;
+  let previousToolMode: ToolMode | null = null;
+
+  return {
+    id: createOpaqueId("command"),
+    label: options.label ?? createDefaultTerrainCommandLabel(true),
+    execute(context) {
+      const currentDocument = context.getDocument();
+      const currentTerrain = currentDocument.terrains[nextTerrain.id];
+
+      if (previousSelection === null) {
+        previousSelection = cloneEditorSelection(context.getSelection());
+      }
+
+      if (previousToolMode === null) {
+        previousToolMode = context.getToolMode();
+      }
+
+      if (previousTerrain === null && currentTerrain !== undefined) {
+        previousTerrain = cloneTerrain(currentTerrain);
+      }
+
+      context.setDocument({
+        ...currentDocument,
+        terrains: {
+          ...currentDocument.terrains,
+          [nextTerrain.id]: cloneTerrain(nextTerrain)
+        }
+      });
+      context.setSelection(setSingleTerrainSelection(nextTerrain.id));
+      context.setToolMode("select");
+    },
+    undo(context) {
+      const currentDocument = context.getDocument();
+      const nextTerrains = {
+        ...currentDocument.terrains
+      };
+
+      if (previousTerrain === null) {
+        delete nextTerrains[nextTerrain.id];
+      } else {
+        nextTerrains[nextTerrain.id] = cloneTerrain(previousTerrain);
+      }
+
+      context.setDocument({
+        ...currentDocument,
+        terrains: nextTerrains
+      });
+
+      if (previousSelection !== null) {
+        context.setSelection(previousSelection);
+      }
+
+      if (previousToolMode !== null) {
+        context.setToolMode(previousToolMode);
+      }
+    }
+  };
+}