@@ -10,6 +10,7 @@
 //------------------------------------------------------------------------------
 
 const astUtils = require("./utils/ast-utils");
+const FixTracker = require("./utils/fix-tracker");
 
 //------------------------------------------------------------------------------
 // Types
@@ -460,9 +461,18 @@ module.exports = {
 						filter: t => t.value === "var",
 					});
 
-					return canFix(node)
-						? fixer.replaceText(varToken, "let")
-						: null;
+					if (!canFix(node)) {
+						return null;
+					}
+
+					/**
+					 * Extend the replacement range to the whole declaration,
+					 * so other fixes in the same declaration wait for the next pass
+					 * and `canFix` judges the declaration they produce.
+					 */
+					return new FixTracker(fixer, sourceCode)
+						.retainRange(node.range)
+						.replaceTextRange(varToken.range, "let");
 				},
 			});
 		}
 
@@ -588,6 +588,18 @@ ruleTester.run("no-var", rule, {
 			output: "function wrap() { function foo() {} foo(); if (bar) { let a = 1; function foo() { console.log(a); } foo (); } }",
 			errors: [{ messageId: "unexpectedVar" }],
 		},
+
+		// https://github.com/eslint/eslint/issues/21387
+		{
+			code: "/*eslint one-var:error*/ function f(a) { var b = 1; var a; }",
+			output: "/*eslint one-var:error*/ function f(a) { let b = 1; var a; }",
+			errors: 3,
+		},
+		{
+			code: "/*eslint one-var:error*/ function f() { var b = 1; var c = 2; }",
+			output: "/*eslint one-var:error*/ function f() { let b = 1; let c = 2; }",
+			errors: 3,
+		},
 	],
 });