diff --git a/src/WindowedSelect.tsx b/src/WindowedSelect.tsx
index d2919ca..cca8f47 100644
--- a/src/WindowedSelect.tsx
+++ b/src/WindowedSelect.tsx
@@ -1,24 +1,186 @@
 import MenuList from './MenuList';
 import * as React from 'react';
-import Select, { Props as SelectProps } from 'react-select';
-import { calcOptionsLength } from './util';
+import Select, { Props as SelectProps, GroupBase, components as defaultComponents } from 'react-select';
+import { calcOptionsLength, flattenOptions } from './util';
+
+interface SelectAllConfig {
+  /** Label shown when not all options are selected. Default: "Select All" */
+  selectAllLabel?: string
+  /** Label shown when all options are selected. Default: "Deselect All" */
+  deselectAllLabel?: string
+}
 
 interface WindowedSelectProps extends SelectProps {
   windowThreshold: number
+  /**
+   * Enable a "Select All" / "Deselect All" option at the top of the menu.
+   * Only works when `isMulti` is true.
+   * Pass `true` for default labels, or an object to customize labels.
+   */
+  enableSelectAll?: boolean | SelectAllConfig
 }
 
-function WindowedSelect ({ windowThreshold = 100, ...passedProps }: WindowedSelectProps, ref) {
+const SELECT_ALL_VALUE = '__windowed_select_all__';
+
+function isSelectAllOption(option: any): boolean {
+  return option && option.value === SELECT_ALL_VALUE;
+}
+
+function WindowedSelect ({ windowThreshold = 100, enableSelectAll, ...passedProps }: WindowedSelectProps, ref) {
+  const { options, isMulti, value, onChange, filterOption } = passedProps;
+  const [inputValue, setInputValue] = React.useState('');
+
   const optionsLength = React.useMemo(
-    () => calcOptionsLength(passedProps.options),
-    [passedProps.options]
+    () => calcOptionsLength(options),
+    [options]
   );
   const isWindowed = optionsLength >= windowThreshold;
 
+  // Flatten grouped options for comparison
+  const allFlatOptions = React.useMemo(
+    () => flattenOptions(options || []),
+    [options]
+  );
+
+  // Compute filtered options based on current search input
+  const filteredOptions = React.useMemo(() => {
+    if (!inputValue) return allFlatOptions;
+
+    return allFlatOptions.filter(option => {
+      if (filterOption) {
+        return filterOption(
+          { label: option.label, value: option.value, data: option },
+          inputValue
+        );
+      }
+      // Default: case-insensitive label match
+      return String(option.label || '').toLowerCase().includes(inputValue.toLowerCase());
+    });
+  }, [allFlatOptions, inputValue, filterOption]);
+
+  const shouldShowSelectAll = enableSelectAll && isMulti;
+
+  // Determine if all filtered options are currently selected
+  const selectedValues = React.useMemo(() => {
+    if (!Array.isArray(value)) return new Set();
+    const getOptValue = passedProps.getOptionValue || ((opt: any) => opt.value);
+    return new Set(value.map(v => getOptValue(v)));
+  }, [value, passedProps.getOptionValue]);
+
+  const allFilteredSelected = React.useMemo(() => {
+    if (!filteredOptions.length) return false;
+    const getOptValue = passedProps.getOptionValue || ((opt: any) => opt.value);
+    return filteredOptions.every(opt => selectedValues.has(getOptValue(opt)));
+  }, [filteredOptions, selectedValues, passedProps.getOptionValue]);
+
+  // Build select all option
+  const selectAllOption = React.useMemo(() => {
+    if (!shouldShowSelectAll) return null;
+
+    const config: SelectAllConfig = typeof enableSelectAll === 'object' ? enableSelectAll : {};
+    const label = allFilteredSelected
+      ? (config.deselectAllLabel || 'Deselect All')
+      : (config.selectAllLabel || 'Select All');
+
+    return { label, value: SELECT_ALL_VALUE };
+  }, [shouldShowSelectAll, allFilteredSelected, enableSelectAll]);
+
+  // Prepend select all option to the options list
+  const enhancedOptions = React.useMemo(() => {
+    if (!selectAllOption || !options) return options;
+
+    // Handle grouped options
+    const head = (options as any[])[0] || {};
+    if (head.options !== undefined) {
+      // Grouped: insert as a standalone group at the top
+      return [
+        { label: '', options: [selectAllOption] } as GroupBase<any>,
+        ...(options as GroupBase<any>[])
+      ];
+    }
+
+    return [selectAllOption, ...(options as any[])];
+  }, [selectAllOption, options]);
+
+  // Intercept onChange to handle select all toggle
+  const handleChange = React.useCallback((selected: any, actionMeta: any) => {
+    if (!onChange) return;
+
+    if (shouldShowSelectAll && actionMeta.action === 'select-option' && isSelectAllOption(actionMeta.option)) {
+      if (allFilteredSelected) {
+        // Deselect all filtered options, keep options that aren't in the filtered set
+        const getOptValue = passedProps.getOptionValue || ((opt: any) => opt.value);
+        const filteredValueSet = new Set(filteredOptions.map(opt => getOptValue(opt)));
+        const remaining = Array.isArray(value)
+          ? value.filter(v => !filteredValueSet.has(getOptValue(v)))
+          : [];
+        onChange(remaining, { ...actionMeta, action: 'deselect-option' });
+      } else {
+        // Select all filtered options, merged with already selected
+        const getOptValue = passedProps.getOptionValue || ((opt: any) => opt.value);
+        const currentSelected = Array.isArray(value) ? [...value] : [];
+        const newOptions = filteredOptions.filter(
+          opt => !selectedValues.has(getOptValue(opt))
+        );
+        onChange([...currentSelected, ...newOptions], actionMeta);
+      }
+      return;
+    }
+
+    // Filter out the select-all option from the selected values
+    if (shouldShowSelectAll && Array.isArray(selected)) {
+      selected = selected.filter(opt => !isSelectAllOption(opt));
+    }
+
+    onChange(selected, actionMeta);
+  }, [onChange, shouldShowSelectAll, allFilteredSelected, filteredOptions, value, selectedValues, passedProps.getOptionValue]);
+
+  // Track input value for filtering
+  const handleInputChange = React.useCallback((newInputValue: string, actionMeta: any) => {
+    setInputValue(newInputValue);
+    if (passedProps.onInputChange) {
+      passedProps.onInputChange(newInputValue, actionMeta);
+    }
+  }, [passedProps.onInputChange]);
+
+  // Custom Option component to style the select-all row differently
+  const SelectAllOptionComponent = React.useMemo(() => {
+    const UserOption = passedProps.components?.Option || defaultComponents.Option;
+
+    return (props: any) => {
+      if (isSelectAllOption(props.data)) {
+        return (
+          <UserOption
+            {...props}
+            isSelected={allFilteredSelected}
+          >
+            <span style={{ fontWeight: 500 }}>{props.data.label}</span>
+          </UserOption>
+        );
+      }
+      return <UserOption {...props} />;
+    };
+  }, [passedProps.components?.Option, allFilteredSelected]);
+
+  // Hide the select-all option from the selected values display
+  const isOptionSelected = React.useCallback((option: any, selectValue: any) => {
+    if (isSelectAllOption(option)) return allFilteredSelected;
+    if (passedProps.isOptionSelected) return passedProps.isOptionSelected(option, selectValue);
+
+    const getOptValue = passedProps.getOptionValue || ((opt: any) => opt.value);
+    return selectedValues.has(getOptValue(option));
+  }, [allFilteredSelected, selectedValues, passedProps.isOptionSelected, passedProps.getOptionValue]);
+
   return (
     <Select
       {...passedProps}
+      options={shouldShowSelectAll ? enhancedOptions : passedProps.options}
+      onChange={handleChange}
+      onInputChange={handleInputChange}
+      isOptionSelected={shouldShowSelectAll ? isOptionSelected : passedProps.isOptionSelected}
       components={{
         ...passedProps.components,
+        ...(shouldShowSelectAll ? { Option: SelectAllOptionComponent } : {}),
         ...(
           isWindowed
             ? { MenuList }
@@ -30,4 +192,4 @@ function WindowedSelect ({ windowThreshold = 100, ...passedProps }: WindowedSele
   );
 }
 
-export default React.forwardRef(WindowedSelect);
\ No newline at end of file
+export default React.forwardRef(WindowedSelect);
diff --git a/src/util.ts b/src/util.ts
index 328977c..59d032c 100644
--- a/src/util.ts
+++ b/src/util.ts
@@ -10,6 +10,19 @@ export function calcOptionsLength (options) {
     : options.length;
 }
 
+export function flattenOptions (options: readonly any[]): any[] {
+  const head = options[0] || {};
+  const isGrouped = head.options !== undefined;
+
+  if (isGrouped) {
+    return (options as any[]).reduce((result: any[], group: any) => {
+      return [...result, ...(group.options || [])];
+    }, []);
+  }
+
+  return [...options];
+}
+
 export function flattenGroupedChildren(children) {
   return children.reduce((result, child) => {
     if (child.props.children != null && typeof child.props.children === "string") {