diff --git a/packages/react-devtools-shared/src/devtools/views/Components/Components.css b/packages/react-devtools-shared/src/devtools/views/Components/Components.css
index f37b3eaa73d3f..5db5eaea2d667 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/Components.css
+++ b/packages/react-devtools-shared/src/devtools/views/Components/Components.css
@@ -1,36 +1,42 @@
-.Components {
-  position: relative;
-  width: 100%;
-  height: 100%;
-  display: flex;
-  flex-direction: row;
-  background-color: var(--color-background);
-  color: var(--color-text);
-  font-family: var(--font-family-sans);
-}
-
 .TreeWrapper {
-  flex: 0 0 65%;
+  flex: 0 0 var(--horizontal-resize-percentage);
   overflow: auto;
 }
 
 .SelectedElementWrapper {
-  flex: 0 0 35%;
+  flex: 1 1 35%;
   overflow-x: hidden;
   overflow-y: auto;
 }
 
-@media screen and (max-width: 600px) {
-  .Components {
-    flex-direction: column;
-  }
+.ResizeBarWrapper {
+  flex: 0 0 0px;
+  position: relative;
+}
+
+.ResizeBar {
+  position: absolute;
+  left: -2px;
+  width: 5px;
+  height: 100%;
+  cursor: ew-resize;
+}
 
+@media screen and (max-width: 600px) {
   .TreeWrapper {
-    flex: 0 0 50%;
+    flex: 0 0 var(--vertical-resize-percentage);
   }
 
   .SelectedElementWrapper {
-    flex: 0 0 50%;
+    flex: 1 1 50%;
+  }
+
+  .ResizeBar {
+    top: -2px;
+    left: 0;
+    width: 100%;
+    height: 5px;
+    cursor: ns-resize;
   }
 }
 
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/Components.js b/packages/react-devtools-shared/src/devtools/views/Components/Components.js
index 6b65e0d2035f9..af1cf3ac95f7b 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/Components.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/Components.js
@@ -8,12 +8,13 @@
  */
 
 import * as React from 'react';
-import {Suspense} from 'react';
+import {Suspense, Fragment} from 'react';
 import Tree from './Tree';
 import SelectedElement from './SelectedElement';
 import {InspectedElementContextController} from './InspectedElementContext';
 import {NativeStyleContextController} from './NativeStyleEditor/context';
 import {OwnersListContextController} from './OwnersListContext';
+import ComponentsResizer from './ComponentsResizer';
 import portaledContent from '../portaledContent';
 import {ModalDialog} from '../ModalDialog';
 import SettingsModal from 'react-devtools-shared/src/devtools/views/Settings/SettingsModal';
@@ -22,25 +23,34 @@ import {SettingsModalContextController} from 'react-devtools-shared/src/devtools
 import styles from './Components.css';
 
 function Components(_: {||}) {
-  // TODO Flex wrappers below should be user resizable.
   return (
     <SettingsModalContextController>
       <OwnersListContextController>
         <InspectedElementContextController>
-          <div className={styles.Components}>
-            <div className={styles.TreeWrapper}>
-              <Tree />
-            </div>
-            <div className={styles.SelectedElementWrapper}>
-              <NativeStyleContextController>
-                <Suspense fallback={<Loading />}>
-                  <SelectedElement />
-                </Suspense>
-              </NativeStyleContextController>
-            </div>
-            <ModalDialog />
-            <SettingsModal />
-          </div>
+          <ComponentsResizer>
+            {({resizeElementRef, onResizeStart}) => (
+              <Fragment>
+                <div ref={resizeElementRef} className={styles.TreeWrapper}>
+                  <Tree />
+                </div>
+                <div className={styles.ResizeBarWrapper}>
+                  <div
+                    onMouseDown={onResizeStart}
+                    className={styles.ResizeBar}
+                  />
+                </div>
+                <div className={styles.SelectedElementWrapper}>
+                  <NativeStyleContextController>
+                    <Suspense fallback={<Loading />}>
+                      <SelectedElement />
+                    </Suspense>
+                  </NativeStyleContextController>
+                </div>
+                <ModalDialog />
+                <SettingsModal />
+              </Fragment>
+            )}
+          </ComponentsResizer>
         </InspectedElementContextController>
       </OwnersListContextController>
     </SettingsModalContextController>
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/ComponentsResizer.css b/packages/react-devtools-shared/src/devtools/views/Components/ComponentsResizer.css
new file mode 100644
index 0000000000000..8a86aa1246a16
--- /dev/null
+++ b/packages/react-devtools-shared/src/devtools/views/Components/ComponentsResizer.css
@@ -0,0 +1,16 @@
+.ComponentsWrapper {
+  position: relative;
+  width: 100%;
+  height: 100%;
+  display: flex;
+  flex-direction: row;
+  background-color: var(--color-background);
+  color: var(--color-text);
+  font-family: var(--font-family-sans);
+}
+
+@media screen and (max-width: 600px) {
+  .ComponentsWrapper {
+    flex-direction: column;
+  }
+}
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/ComponentsResizer.js b/packages/react-devtools-shared/src/devtools/views/Components/ComponentsResizer.js
new file mode 100644
index 0000000000000..f97466843ea59
--- /dev/null
+++ b/packages/react-devtools-shared/src/devtools/views/Components/ComponentsResizer.js
@@ -0,0 +1,213 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+import * as React from 'react';
+import {useEffect, useLayoutEffect, useReducer, useRef} from 'react';
+import {
+  localStorageGetItem,
+  localStorageSetItem,
+} from 'react-devtools-shared/src/storage';
+import styles from './ComponentsResizer.css';
+
+const LOCAL_STORAGE_KEY = 'React::DevTools::createResizeReducer';
+const VERTICAL_MODE_MAX_WIDTH = 600;
+const MINIMUM_SIZE = 50;
+
+type Orientation = 'horizontal' | 'vertical';
+
+type ResizeActionType =
+  | 'ACTION_SET_DID_MOUNT'
+  | 'ACTION_SET_IS_RESIZING'
+  | 'ACTION_SET_HORIZONTAL_PERCENTAGE'
+  | 'ACTION_SET_VERTICAL_PERCENTAGE';
+
+type ResizeAction = {|
+  type: ResizeActionType,
+  payload: any,
+|};
+
+type ResizeState = {|
+  horizontalPercentage: number,
+  isResizing: boolean,
+  verticalPercentage: number,
+|};
+
+function initResizeState(): ResizeState {
+  let horizontalPercentage = 0.65;
+  let verticalPercentage = 0.5;
+
+  try {
+    let data = localStorageGetItem(LOCAL_STORAGE_KEY);
+    if (data != null) {
+      data = JSON.parse(data);
+      horizontalPercentage = data.horizontalPercentage;
+      verticalPercentage = data.verticalPercentage;
+    }
+  } catch (error) {}
+
+  return {
+    horizontalPercentage,
+    isResizing: false,
+    verticalPercentage,
+  };
+}
+
+function resizeReducer(state: ResizeState, action: ResizeAction): ResizeState {
+  switch (action.type) {
+    case 'ACTION_SET_IS_RESIZING':
+      return {
+        ...state,
+        isResizing: action.payload,
+      };
+    case 'ACTION_SET_HORIZONTAL_PERCENTAGE':
+      return {
+        ...state,
+        horizontalPercentage: action.payload,
+      };
+    case 'ACTION_SET_VERTICAL_PERCENTAGE':
+      return {
+        ...state,
+        verticalPercentage: action.payload,
+      };
+    default:
+      return state;
+  }
+}
+
+type Props = {|
+  children: ({
+    resizeElementRef: React$Ref<HTMLElement>,
+    onResizeStart: () => void,
+  }) => React$Node,
+|};
+
+export default function ComponentsResizer({children}: Props) {
+  const wrapperElementRef = useRef<HTMLElement>(null);
+  const resizeElementRef = useRef<HTMLElement>(null);
+
+  const [state, dispatch] = useReducer<ResizeState, ResizeAction>(
+    resizeReducer,
+    null,
+    initResizeState,
+  );
+
+  const {horizontalPercentage, verticalPercentage} = state;
+
+  useLayoutEffect(() => {
+    const resizeElement = resizeElementRef.current;
+
+    setResizeCSSVariable(
+      resizeElement,
+      'horizontal',
+      horizontalPercentage * 100,
+    );
+    setResizeCSSVariable(resizeElement, 'vertical', verticalPercentage * 100);
+  }, []);
+
+  useEffect(() => {
+    const timeoutID = setTimeout(() => {
+      localStorageSetItem(
+        LOCAL_STORAGE_KEY,
+        JSON.stringify({
+          horizontalPercentage,
+          verticalPercentage,
+        }),
+      );
+    }, 500);
+
+    return () => clearTimeout(timeoutID);
+  }, [horizontalPercentage, verticalPercentage]);
+
+  const {isResizing} = state;
+
+  const onResizeStart = () =>
+    dispatch({type: 'ACTION_SET_IS_RESIZING', payload: true});
+  const onResizeEnd = () =>
+    dispatch({type: 'ACTION_SET_IS_RESIZING', payload: false});
+
+  const onResize = event => {
+    const resizeElement = resizeElementRef.current;
+    const wrapperElement = wrapperElementRef.current;
+
+    if (!isResizing || wrapperElement === null || resizeElement === null) {
+      return;
+    }
+
+    event.preventDefault();
+
+    const orientation = getOrientation(wrapperElement);
+
+    const {height, width, left, top} = wrapperElement.getBoundingClientRect();
+
+    const currentMousePosition =
+      orientation === 'horizontal' ? event.clientX - left : event.clientY - top;
+
+    const boundaryMin = MINIMUM_SIZE;
+    const boundaryMax =
+      orientation === 'horizontal'
+        ? width - MINIMUM_SIZE
+        : height - MINIMUM_SIZE;
+
+    const isMousePositionInBounds =
+      currentMousePosition > boundaryMin && currentMousePosition < boundaryMax;
+
+    if (isMousePositionInBounds) {
+      const resizedElementDimension =
+        orientation === 'horizontal' ? width : height;
+      const actionType =
+        orientation === 'horizontal'
+          ? 'ACTION_SET_HORIZONTAL_PERCENTAGE'
+          : 'ACTION_SET_VERTICAL_PERCENTAGE';
+      const percentage = (currentMousePosition / resizedElementDimension) * 100;
+
+      setResizeCSSVariable(resizeElement, orientation, percentage);
+
+      dispatch({
+        type: actionType,
+        payload: currentMousePosition / resizedElementDimension,
+      });
+    }
+  };
+
+  return (
+    <div
+      ref={wrapperElementRef}
+      className={styles.ComponentsWrapper}
+      {...(isResizing && {
+        onMouseMove: onResize,
+        onMouseLeave: onResizeEnd,
+        onMouseUp: onResizeEnd,
+      })}>
+      {children({resizeElementRef, onResizeStart})}
+    </div>
+  );
+}
+
+function getOrientation(
+  wrapperElement: null | HTMLElement,
+): null | Orientation {
+  if (wrapperElement != null) {
+    const {width} = wrapperElement.getBoundingClientRect();
+    return width > VERTICAL_MODE_MAX_WIDTH ? 'horizontal' : 'vertical';
+  }
+  return null;
+}
+
+function setResizeCSSVariable(
+  resizeElement: null | HTMLElement,
+  orientation: null | Orientation,
+  percentage: number,
+): void {
+  if (resizeElement !== null && orientation !== null) {
+    resizeElement.style.setProperty(
+      `--${orientation}-resize-percentage`,
+      `${percentage}%`,
+    );
+  }
+}