diff --git a/src/commands/upsert-entity-command.ts b/src/commands/upsert-entity-command.ts
new file mode 100644
index 00000000..15ce6d17
--- /dev/null
+++ b/src/commands/upsert-entity-command.ts
@@ -0,0 +1,95 @@
+import { cloneEditorSelection, type EditorSelection } from "../core/selection";
+import type { ToolMode } from "../core/tool-mode";
+import { createOpaqueId } from "../core/ids";
+import { cloneEntityInstance, getEntityKindLabel, type EntityInstance } from "../entities/entity-instances";
+
+import type { EditorCommand } from "./command";
+
+interface UpsertEntityCommandOptions {
+  entity: EntityInstance;
+  label?: string;
+}
+
+function setSingleEntitySelection(entityId: string): EditorSelection {
+  return {
+    kind: "entities",
+    ids: [entityId]
+  };
+}
+
+function createDefaultEntityCommandLabel(entity: EntityInstance, isNewEntity: boolean): string {
+  const action = isNewEntity ? "Place" : "Update";
+  return `${action} ${getEntityKindLabel(entity.kind).toLowerCase()}`;
+}
+
+export function createUpsertEntityCommand(options: UpsertEntityCommandOptions): EditorCommand {
+  const nextEntity = cloneEntityInstance(options.entity);
+  let previousEntity = null as EntityInstance | null;
+  let previousSelection: EditorSelection | null = null;
+  let previousToolMode: ToolMode | null = null;
+
+  return {
+    id: createOpaqueId("command"),
+    label: options.label ?? createDefaultEntityCommandLabel(nextEntity, true),
+    execute(context) {
+      const currentDocument = context.getDocument();
+      const currentEntity = currentDocument.entities[nextEntity.id];
+      const isNewEntity = currentEntity === undefined;
+
+      if (currentEntity !== undefined && currentEntity.kind !== nextEntity.kind) {
+        throw new Error(`Entity ${nextEntity.id} is a ${currentEntity.kind}, not a ${nextEntity.kind}.`);
+      }
+
+      if (previousSelection === null) {
+        previousSelection = cloneEditorSelection(context.getSelection());
+      }
+
+      if (previousToolMode === null) {
+        previousToolMode = context.getToolMode();
+      }
+
+      if (previousEntity === null && currentEntity !== undefined) {
+        previousEntity = cloneEntityInstance(currentEntity);
+      }
+
+      context.setDocument({
+        ...currentDocument,
+        entities: {
+          ...currentDocument.entities,
+          [nextEntity.id]: cloneEntityInstance(nextEntity)
+        }
+      });
+      context.setSelection(setSingleEntitySelection(nextEntity.id));
+      context.setToolMode("select");
+
+      if (options.label === undefined) {
+        this.label = createDefaultEntityCommandLabel(nextEntity, isNewEntity);
+      }
+    },
+    undo(context) {
+      const currentDocument = context.getDocument();
+      const nextEntities = {
+        ...currentDocument.entities
+      };
+
+      if (previousEntity === null) {
+        delete nextEntities[nextEntity.id];
+      } else {
+        nextEntities[nextEntity.id] = cloneEntityInstance(previousEntity);
+      }
+
+      context.setDocument({
+        ...currentDocument,
+        entities: nextEntities
+      });
+
+      if (previousSelection !== null) {
+        context.setSelection(previousSelection);
+      }
+
+      if (previousToolMode !== null) {
+        context.setToolMode(previousToolMode);
+      }
+    }
+  };
+}