diff --git a/src/Bundle.js b/src/Bundle.js
index 0cfe895..26e6229 100644
--- a/src/Bundle.js
+++ b/src/Bundle.js
@@ -293,7 +293,7 @@ export default class Bundle {
 					return this.cachedModules.get( id );
 				}
 
-				return transform( source, id, this.plugins );
+				return transform( this, source, id, this.plugins );
 			})
 			.then( source => {
 				const { code, originalCode, originalSourceMap, ast, sourceMapChain, resolvedIds } = source;
diff --git a/src/utils/transform.js b/src/utils/transform.js
index 7e90a74..0dd813c 100644
--- a/src/utils/transform.js
+++ b/src/utils/transform.js
@@ -1,8 +1,10 @@
 import { decode } from 'sourcemap-codec';
+import { locate } from 'locate-character';
 import error from './error.js';
 import relativeId from './relativeId.js';
+import getCodeFrame from './getCodeFrame.js';
 
-export default function transform ( source, id, plugins ) {
+export default function transform ( bundle, source, id, plugins ) {
 	const sourceMapChain = [];
 
 	const originalSourceMap = typeof source.map === 'string' ? JSON.parse( source.map ) : source.map;
@@ -19,7 +21,27 @@ export default function transform ( source, id, plugins ) {
 		return promise.then( previous => {
 			if ( !plugin.transform ) return previous;
 
-			return Promise.resolve( plugin.transform( previous, id ) ).then( result => {
+			const context = {
+				warn: ( warning, pos ) => {
+					if ( typeof warning === 'string' ) {
+						warning = { message: warning };
+					}
+
+					warning.plugin = plugin.name;
+					if ( !warning.code ) warning.code = 'PLUGIN_WARNING';
+
+					if ( pos !== undefined ) {
+						warning.pos = pos;
+						const { line, column } = locate( previous, pos, { offsetLine: 1 });
+						warning.loc = { file: id, line, column };
+						warning.frame = getCodeFrame( previous, line, column );
+					}
+
+					bundle.warn( warning );
+				}
+			};
+
+			return Promise.resolve( plugin.transform.call( context, previous, id ) ).then( result => {
 				if ( result == null ) return previous;
 
 				if ( typeof result === 'string' ) {
@@ -29,6 +51,7 @@ export default function transform ( source, id, plugins ) {
 						map: null
 					};
 				}
+
 				// `result.map` can only be a string if `result` isn't
 				else if ( typeof result.map === 'string' ) {
 					result.map = JSON.parse( result.map );
diff --git a/test/function/plugin-warn/_config.js b/test/function/plugin-warn/_config.js
new file mode 100644
index 0000000..1fd2f8c
--- /dev/null
+++ b/test/function/plugin-warn/_config.js
@@ -0,0 +1,37 @@
+const path = require( 'path' );
+
+module.exports = {
+	description: 'plugin transform hooks can use `this.warn({...}, char)` (#1140)',
+	options: {
+		plugins: [{
+			name: 'test',
+			transform ( code, id ) {
+				this.warn({ message: 'foo' });
+				this.warn( 'bar', 22 );
+				return 'assert.equal( 21 * 2, 42 );';
+			}
+		}]
+	},
+	warnings: [
+		{
+			code: 'PLUGIN_WARNING',
+			plugin: 'test',
+			message: 'foo'
+		},
+		{
+			code: 'PLUGIN_WARNING',
+			plugin: 'test',
+			message: 'bar',
+			pos: 22,
+			loc: {
+				file: path.resolve( __dirname, 'main.js' ),
+				line: 1,
+				column: 22
+			},
+			frame: `
+				1: assert.equal( 21 * 2, TK );
+				                         ^
+			`
+		}
+	]
+};
diff --git a/test/function/plugin-warn/main.js b/test/function/plugin-warn/main.js
new file mode 100644
index 0000000..04164ee
--- /dev/null
+++ b/test/function/plugin-warn/main.js
@@ -0,0 +1 @@
+assert.equal( 21 * 2, TK );