@@ -13,6 +13,8 @@
 
 -   `Text`: Make single-line truncation and `isBlock` overrides consistent across the main document and iframes ([#82573](https://github.com/WordPress/gutenberg/pull/82573)).
 -   `CardDivider`: Apply the Card's width, display, and border color consistently inside iframes ([#82577](https://github.com/WordPress/gutenberg/pull/82577)).
+-   `BorderControl`, `BorderBoxControl`: Stop offering `%` as a width unit, since `border-width` does not accept percentages. A width already set in `%` keeps showing its unit ([#83492](https://github.com/WordPress/gutenberg/pull/83492)).
+-   `UnitControl`: After clearing a value whose unit is not in `units`, give the next typed value the unit shown in the select instead of the cleared one ([#83492](https://github.com/WordPress/gutenberg/pull/83492)).
 
 ### Internal
 
 
@@ -1,6 +1,7 @@
 import { __ } from '@wordpress/i18n';
 import BorderControlDropdown from '../border-control-dropdown';
 import UnitControl from '../../unit-control';
+import { CSS_UNITS } from '../../unit-control/utils';
 import RangeControl from '../../range-control';
 import { HStack } from '../../h-stack';
 import { StyledLabel } from '../../base-control/styles/base-control-styles';
@@ -12,6 +13,9 @@ import { useBorderControl } from './hook';
 import type { BorderControlProps, LabelProps } from '../types';
 import { Spacer } from '../../spacer';
 
+// Percentages are not valid `border-width` values.
+const BORDER_WIDTH_UNITS = CSS_UNITS.filter( ( unit ) => unit.value !== '%' );
+
 const BorderLabel = ( props: LabelProps ) => {
 	const { label, hideLabelFromVision } = props;
 
@@ -93,6 +97,7 @@ const UnconnectedBorderControl = (
 					onChange={ onWidthChange }
 					value={ border?.width || '' }
 					placeholder={ placeholder }
+					units={ BORDER_WIDTH_UNITS }
 					disableUnits={ disableUnits }
 					__unstableInputWidth={ inputWidth }
 				/>
@@ -105,7 +110,7 @@ const UnconnectedBorderControl = (
 						max={ 100 }
 						min={ 0 }
 						onChange={ onSliderChange }
-						step={ [ 'px', '%' ].includes( widthUnit ) ? 1 : 0.1 }
+						step={ widthUnit === 'px' ? 1 : 0.1 }
 						value={ widthValue || undefined }
 						withInputField={ false }
 					/>
 
@@ -139,6 +139,44 @@ describe( 'BorderControl', () => {
 			expect( widthInput ).toHaveAttribute( 'placeholder', 'Mixed' );
 		} );
 
+		it( 'should not offer a percentage unit for the width', () => {
+			const props = createProps();
+			render( <TestBorderControl { ...props } /> );
+
+			const unitSelect = screen.getByRole( 'combobox', {
+				name: 'Select unit',
+			} );
+			expect(
+				within( unitSelect ).queryByRole( 'option', { name: '%' } )
+			).not.toBeInTheDocument();
+		} );
+
+		it( 'should keep the unit of a percentage width until it is changed', async () => {
+			const user = userEvent.setup();
+			const props = createProps( {
+				value: { ...defaultBorder, width: '10%' },
+			} );
+			const { rerender } = render( <TestBorderControl { ...props } /> );
+
+			const unitSelect = screen.getByRole( 'combobox', {
+				name: 'Select unit',
+			} );
+			expect( unitSelect ).toHaveValue( '%' );
+
+			await user.selectOptions( unitSelect, 'px' );
+
+			expect( props.onChange ).toHaveBeenNthCalledWith( 1, {
+				...defaultBorder,
+				width: '10px',
+			} );
+
+			rerender( <TestBorderControl { ...props } /> );
+
+			expect(
+				within( unitSelect ).queryByRole( 'option', { name: '%' } )
+			).not.toBeInTheDocument();
+		} );
+
 		describe( 'color indicator inline styles', () => {
 			const getIndicatorWrapper = ( border: Border ) => {
 				render(
 
@@ -119,6 +119,12 @@ function UnforwardedUnitControl(
 			return;
 		}
 
+		// After clearing a value whose unit isn't offered, the select shows the
+		// first offered unit, so only fall back to the current unit if offered.
+		const fallbackUnit = units.some( ( option ) => option.value === unit )
+			? unit
+			: undefined;
+
 		/*
 		 * Customizing the onChange callback.
 		 * This allows as to broadcast a combined value+unit to onChange.
@@ -127,7 +133,7 @@ function UnforwardedUnitControl(
 			nextQuantityValue,
 			units,
 			parsedQuantity,
-			unit
+			fallbackUnit
 		).join( '' );
 
 		onChangeProp?.( onChangeValue, changeProps );
 
@@ -529,6 +529,43 @@ describe( 'UnitControl', () => {
 			expect( select ).toHaveValue( 'vmax' );
 		} );
 
+		it( 'should use the unit shown in the select after clearing a value whose unit is not offered', async () => {
+			const user = userEvent.setup();
+			const onChangeSpy = vi.fn();
+
+			const ControlledUnitControl = () => {
+				const [ value, setValue ] = useState< string | undefined >(
+					'10%'
+				);
+				return (
+					<UnitControl
+						value={ value }
+						units={ [
+							{ value: 'px', label: 'px' },
+							{ value: 'em', label: 'em' },
+						] }
+						onChange={ ( nextValue, extra ) => {
+							setValue( nextValue );
+							onChangeSpy( nextValue, extra );
+						} }
+					/>
+				);
+			};
+			render( <ControlledUnitControl /> );
+
+			const input = getInput();
+			await user.clear( input );
+
+			expect( getSelect() ).toHaveValue( 'px' );
+
+			await user.type( input, '5' );
+
+			expect( onChangeSpy ).toHaveBeenLastCalledWith(
+				'5px',
+				expect.anything()
+			);
+		} );
+
 		it( 'should run onBlur callback when the unit select is blurred', async () => {
 			const user = userEvent.setup();