diff --git a/packages/runtime-core/src/component.ts b/packages/runtime-core/src/component.ts
index 57a53a39b76..93169008fe7 100644
--- a/packages/runtime-core/src/component.ts
+++ b/packages/runtime-core/src/component.ts
@@ -224,6 +224,10 @@ export interface ComponentInternalInstance {
   parent: ComponentInternalInstance | null
   root: ComponentInternalInstance
   appContext: AppContext
+  /**
+   * ShadowRoot of custom element if it is
+   */
+  container: ShadowRoot | null
   /**
    * Vnode representing this component in its parent's vdom tree
    */
@@ -497,6 +501,7 @@ export function createComponentInstance(
     type,
     parent,
     appContext,
+    container: null, // will be set synchronously before setup
     root: null!, // to be immediately set
     next: null,
     subTree: null!, // will be set synchronously right after creation
diff --git a/packages/runtime-core/src/renderer.ts b/packages/runtime-core/src/renderer.ts
index 383e17fb0f5..ef1e24cc9b1 100644
--- a/packages/runtime-core/src/renderer.ts
+++ b/packages/runtime-core/src/renderer.ts
@@ -1201,6 +1201,7 @@ function baseCreateRenderer(
         parentComponent,
         parentSuspense
       ))
+    instance.isCE && (instance.container = container as ShadowRoot)
 
     if (__DEV__ && instance.type.__hmrId) {
       registerHMR(instance)
@@ -2237,6 +2238,8 @@ function baseCreateRenderer(
       invokeArrayFns(bum)
     }
 
+    instance.container = null
+
     if (
       __COMPAT__ &&
       isCompatEnabled(DeprecationTypes.INSTANCE_EVENT_HOOKS, instance)
diff --git a/packages/runtime-dom/__tests__/customElement.spec.ts b/packages/runtime-dom/__tests__/customElement.spec.ts
index 6ccf02c4fa6..5b76952d3f6 100644
--- a/packages/runtime-dom/__tests__/customElement.spec.ts
+++ b/packages/runtime-dom/__tests__/customElement.spec.ts
@@ -2,6 +2,7 @@ import {
   defineAsyncComponent,
   defineComponent,
   defineCustomElement,
+  getCurrentInstance,
   h,
   inject,
   nextTick,
@@ -693,4 +694,24 @@ describe('defineCustomElement', () => {
       )
     })
   })
+
+  describe('shadowRoot', () => {
+    // #6113
+    test('shadowRoot accessible for css-in-js', () => {
+      const Foo = defineCustomElement({
+        setup() {
+          const ins = getCurrentInstance()!
+          const style = document.createElement('style')
+          style.innerHTML = `div { color: red; }`
+          ins.container!.appendChild(style)
+          return () => h('div', 'hello')
+        }
+      })
+      customElements.define('my-el', Foo)
+      container.innerHTML = `<my-el></my-el>`
+      const el = container.childNodes[0] as VueElement
+      const style = el.shadowRoot?.querySelector('style')!
+      expect(style.textContent).toBe(`div { color: red; }`)
+    })
+  })
 })