diff --git a/.changeset/block-missing-shopify-attributes.md b/.changeset/block-missing-shopify-attributes.md
new file mode 100644
index 000000000..e66906dfe
--- /dev/null
+++ b/.changeset/block-missing-shopify-attributes.md
@@ -0,0 +1,7 @@
+---
+'@shopify/theme-check-common': minor
+---
+
+Add `BlockMissingShopifyAttributes` check. Warns when a `blocks/*.liquid` file declares `"tag": null` in its `{% schema %}` but its rendered markup does not include `{{ block.shopify_attributes }}`. Without that, the theme editor cannot recognise the block in the preview, and merchants reordering blocks leaves orphaned markup behind. See [Shopify's `tag` field documentation](https://shopify.dev/docs/storefronts/themes/architecture/blocks/theme-blocks/schema#tag).
+
+Phase 1: only the current file is checked. Cases where the markup is delegated to a rendered snippet, or where `block.shopify_attributes` is rendered multiple times, are out of scope.
diff --git a/packages/theme-check-common/src/checks/block-missing-shopify-attributes/index.spec.ts b/packages/theme-check-common/src/checks/block-missing-shopify-attributes/index.spec.ts
new file mode 100644
index 000000000..273b947ff
--- /dev/null
+++ b/packages/theme-check-common/src/checks/block-missing-shopify-attributes/index.spec.ts
@@ -0,0 +1,155 @@
+import { expect, describe, it } from 'vitest';
+import { BlockMissingShopifyAttributes } from './index';
+import { check, MockTheme } from '../../test';
+
+describe('Module: BlockMissingShopifyAttributes', () => {
+  it('reports an offense when the block schema sets `tag: null` and the markup omits `block.shopify_attributes`', async () => {
+    const theme: MockTheme = {
+      'blocks/text.liquid': `
+        <h2>{{ block.settings.heading }}</h2>
+        {% schema %}
+        {
+          "name": "Text",
+          "tag": null,
+          "settings": []
+        }
+        {% endschema %}
+      `,
+    };
+
+    const offenses = await check(theme, [BlockMissingShopifyAttributes]);
+    expect(offenses).toHaveLength(1);
+    expect(offenses[0].message).to.match(/must render `\{\{ block\.shopify_attributes \}\}`/);
+    expect(offenses[0].check).to.eql('BlockMissingShopifyAttributes');
+  });
+
+  it('does not report when `block.shopify_attributes` is rendered on the wrapper', async () => {
+    const theme: MockTheme = {
+      'blocks/text.liquid': `
+        <div {{ block.shopify_attributes }}>
+          <h2>{{ block.settings.heading }}</h2>
+        </div>
+        {% schema %}
+        {
+          "name": "Text",
+          "tag": null,
+          "settings": []
+        }
+        {% endschema %}
+      `,
+    };
+
+    const offenses = await check(theme, [BlockMissingShopifyAttributes]);
+    expect(offenses).to.be.empty;
+  });
+
+  it('does not report when `tag` is omitted (Shopify wraps the block automatically)', async () => {
+    const theme: MockTheme = {
+      'blocks/text.liquid': `
+        <h2>{{ block.settings.heading }}</h2>
+        {% schema %}
+        {
+          "name": "Text",
+          "settings": []
+        }
+        {% endschema %}
+      `,
+    };
+
+    const offenses = await check(theme, [BlockMissingShopifyAttributes]);
+    expect(offenses).to.be.empty;
+  });
+
+  it('does not report when `tag` is a non-null string (Shopify wraps the block in that element)', async () => {
+    const theme: MockTheme = {
+      'blocks/text.liquid': `
+        <h2>{{ block.settings.heading }}</h2>
+        {% schema %}
+        {
+          "name": "Text",
+          "tag": "section",
+          "settings": []
+        }
+        {% endschema %}
+      `,
+    };
+
+    const offenses = await check(theme, [BlockMissingShopifyAttributes]);
+    expect(offenses).to.be.empty;
+  });
+
+  it('accepts bracket access (`block["shopify_attributes"]`) as valid', async () => {
+    const theme: MockTheme = {
+      'blocks/text.liquid': `
+        <div {{ block["shopify_attributes"] }}>content</div>
+        {% schema %}
+        {
+          "name": "Text",
+          "tag": null
+        }
+        {% endschema %}
+      `,
+    };
+
+    const offenses = await check(theme, [BlockMissingShopifyAttributes]);
+    expect(offenses).to.be.empty;
+  });
+
+  it('does not run on section files', async () => {
+    // Even if a section has `tag: null` declared (which is permitted on
+    // sections too), the requirement to render block.shopify_attributes is
+    // a block-specific concern, so this check should ignore sections.
+    const theme: MockTheme = {
+      'sections/missing.liquid': `
+        <div></div>
+        {% schema %}
+        {
+          "name": "Section",
+          "tag": null,
+          "settings": []
+        }
+        {% endschema %}
+      `,
+    };
+
+    const offenses = await check(theme, [BlockMissingShopifyAttributes]);
+    expect(offenses).to.be.empty;
+  });
+
+  it('does not run on snippet files', async () => {
+    // Snippets do not have schemas, so the check has nothing to act on.
+    // This is a guard against future changes that might trip on the path.
+    const theme: MockTheme = {
+      'snippets/helper.liquid': `
+        <div>{{ thing }}</div>
+      `,
+    };
+
+    const offenses = await check(theme, [BlockMissingShopifyAttributes]);
+    expect(offenses).to.be.empty;
+  });
+
+  it('reports the offense at the schema `tag` field position', async () => {
+    const source = `
+        <h2>{{ block.settings.heading }}</h2>
+        {% schema %}
+        {
+          "name": "Text",
+          "tag": null
+        }
+        {% endschema %}
+      `;
+    const theme: MockTheme = {
+      'blocks/text.liquid': source,
+    };
+
+    const offenses = await check(theme, [BlockMissingShopifyAttributes]);
+    expect(offenses).toHaveLength(1);
+
+    // The reported range should be inside the schema body, anchored on the
+    // `tag` field's value (a sanity check that we're using the JSON AST
+    // node positions correctly).
+    const offendingRange = source.slice(offenses[0].start.index, offenses[0].end.index);
+    expect(offendingRange).to.match(/null/);
+  });
+});
diff --git a/packages/theme-check-common/src/checks/block-missing-shopify-attributes/index.ts b/packages/theme-check-common/src/checks/block-missing-shopify-attributes/index.ts
new file mode 100644
index 000000000..ae027aed8
--- /dev/null
+++ b/packages/theme-check-common/src/checks/block-missing-shopify-attributes/index.ts
@@ -0,0 +1,102 @@
+import { LiquidVariableLookup } from '@shopify/liquid-html-parser';
+import { LiquidCheckDefinition, Severity, SourceCodeType } from '../../types';
+import { isBlock } from '../../to-schema';
+import { getSchema } from '../../to-schema';
+import { nodeAtPath } from '../../json';
+import { reportWarning } from '../../utils';
+
+const SHOPIFY_ATTRIBUTES_KEY = 'shopify_attributes';
+const BLOCK_OBJECT = 'block';
+
+/**
+ * When a block schema declares `"tag": null`, the block has no wrapping element
+ * generated by Shopify. To stay registered with the theme editor, the markup
+ * the block renders MUST contain `{{ block.shopify_attributes }}` somewhere on
+ * its outermost element. Forgetting this leaves the block unrecognised in the
+ * preview and produces orphaned markup when merchants reorder blocks.
+ *
+ * @see https://shopify.dev/docs/storefronts/themes/architecture/blocks/theme-blocks/schema#tag
+ *
+ * Phase 1 (this implementation, per #867): only the current file is checked.
+ * Cases where the block's markup is delegated to a rendered snippet, or where
+ * `block.shopify_attributes` ends up rendered multiple times, are out of scope.
+ */
+export const BlockMissingShopifyAttributes: LiquidCheckDefinition = {
+  meta: {
+    code: 'BlockMissingShopifyAttributes',
+    name: 'Block missing block.shopify_attributes',
+    docs: {
+      description:
+        'Warns when a block file with `"tag": null` in its schema does not render `{{ block.shopify_attributes }}` on its outermost element.',
+      recommended: true,
+      url: 'https://shopify.dev/docs/storefronts/themes/tools/theme-check/checks/block-missing-shopify-attributes',
+    },
+    type: SourceCodeType.LiquidHtml,
+    severity: Severity.WARNING,
+    schema: {},
+    targets: [],
+  },
+
+  create(context) {
+    // Skip non-block files entirely. Section files have their own wrapping
+    // semantics and do not require `block.shopify_attributes`.
+    if (!isBlock(context.file.uri)) return {};
+
+    let hasShopifyAttributes = false;
+    let schemaOffset: number | null = null;
+
+    return {
+      async LiquidRawTag(node) {
+        if (node.name !== 'schema' || node.body.kind !== 'json') return;
+        // Captured here so we can translate JSON-AST positions to file
+        // positions inside `onCodePathEnd`.
+        schemaOffset = node.blockStartPosition.end;
+      },
+
+      async VariableLookup(node: LiquidVariableLookup) {
+        if (hasShopifyAttributes) return;
+        if (node.name !== BLOCK_OBJECT) return;
+
+        // `block.shopify_attributes` shows up as a VariableLookup with
+        // `name === 'block'` and a single string lookup of
+        // `'shopify_attributes'`. Bracket access (`block['shopify_attributes']`)
+        // is also accepted because liquid-html-parser normalises both forms
+        // to a `String` lookup with the same `value`.
+        const matches = node.lookups.some(
+          (lookup) => lookup.type === 'String' && lookup.value === SHOPIFY_ATTRIBUTES_KEY,
+        );
+        if (matches) {
+          hasShopifyAttributes = true;
+        }
+      },
+
+      async onCodePathEnd() {
+        if (hasShopifyAttributes) return;
+        if (schemaOffset === null) return;
+
+        const schema = await getSchema(context);
+        if (!schema) return;
+
+        const { validSchema, ast } = schema;
+        if (!validSchema || validSchema instanceof Error) return;
+        if (!ast || ast instanceof Error) return;
+
+        // The schema field is `tag` and we only flag the explicit `null` case.
+        // Missing `tag` (i.e. the schema-default behaviour where Shopify wraps
+        // the block in a `<div>`) does NOT require `block.shopify_attributes`,
+        // because Shopify's wrapper element receives the attributes itself.
+        if (!('tag' in validSchema) || validSchema.tag !== null) return;
+
+        const tagNode = nodeAtPath(ast, ['tag']);
+        if (!tagNode) return;
+
+        reportWarning(
+          `Blocks with \`"tag": null\` must render \`{{ block.shopify_attributes }}\` on their outermost element so the theme editor can recognise the block.`,
+          schemaOffset,
+          tagNode,
+          context,
+        );
+      },
+    };
+  },
+};
diff --git a/packages/theme-check-common/src/checks/index.ts b/packages/theme-check-common/src/checks/index.ts
index be0370c41..7d9bec01d 100644
--- a/packages/theme-check-common/src/checks/index.ts
+++ b/packages/theme-check-common/src/checks/index.ts
@@ -7,6 +7,7 @@ import { AssetSizeAppBlockJavaScript } from './asset-size-app-block-javascript';
 import { AssetSizeCSS } from './asset-size-css';
 import { AssetSizeJavaScript } from './asset-size-javascript';
 import { BlockIdUsage } from './block-id-usage';
+import { BlockMissingShopifyAttributes } from './block-missing-shopify-attributes';
 import { CdnPreconnect } from './cdn-preconnect';
 import { ContentForHeaderModification } from './content-for-header-modification';
 import { DeprecateBgsizes } from './deprecate-bgsizes';
@@ -77,6 +78,7 @@ export const allChecks: (LiquidCheckDefinition | JSONCheckDefinition)[] = [
   AssetSizeCSS,
   AssetSizeJavaScript,
   BlockIdUsage,
+  BlockMissingShopifyAttributes,
   CdnPreconnect,
   ContentForHeaderModification,
   DeprecateBgsizes,
diff --git a/packages/theme-check-node/configs/all.yml b/packages/theme-check-node/configs/all.yml
index 426115894..82a3f1f7d 100644
--- a/packages/theme-check-node/configs/all.yml
+++ b/packages/theme-check-node/configs/all.yml
@@ -31,6 +31,9 @@ AssetSizeJavaScript:
 BlockIdUsage:
   enabled: true
   severity: 1
+BlockMissingShopifyAttributes:
+  enabled: true
+  severity: 1
 CdnPreconnect:
   enabled: true
   severity: 0
diff --git a/packages/theme-check-node/configs/recommended.yml b/packages/theme-check-node/configs/recommended.yml
index adaa03fdf..dd9c1ddff 100644
--- a/packages/theme-check-node/configs/recommended.yml
+++ b/packages/theme-check-node/configs/recommended.yml
@@ -9,6 +9,9 @@ AssetPreload:
 BlockIdUsage:
   enabled: true
   severity: 1
+BlockMissingShopifyAttributes:
+  enabled: true
+  severity: 1
 CdnPreconnect:
   enabled: true
   severity: 0