diff --git a/api_guard/dist/types/operators/index.d.ts b/api_guard/dist/types/operators/index.d.ts
index 8eafe2cf37..664e209916 100644
--- a/api_guard/dist/types/operators/index.d.ts
+++ b/api_guard/dist/types/operators/index.d.ts
@@ -80,8 +80,8 @@ export declare function dematerialize<N extends ObservableNotification<any>>():
 
 export declare function distinct<T, K>(keySelector?: (value: T) => K, flushes?: Observable<any>): MonoTypeOperatorFunction<T>;
 
-export declare function distinctUntilChanged<T>(compare?: (x: T, y: T) => boolean): MonoTypeOperatorFunction<T>;
-export declare function distinctUntilChanged<T, K>(compare: (x: K, y: K) => boolean, keySelector: (x: T) => K): MonoTypeOperatorFunction<T>;
+export declare function distinctUntilChanged<T>(comparator?: (previous: T, current: T) => boolean): MonoTypeOperatorFunction<T>;
+export declare function distinctUntilChanged<T, K>(comparator: (previous: K, current: K) => boolean, keySelector: (value: T) => K): MonoTypeOperatorFunction<T>;
 
 export declare function distinctUntilKeyChanged<T>(key: keyof T): MonoTypeOperatorFunction<T>;
 export declare function distinctUntilKeyChanged<T, K extends keyof T>(key: K, compare: (x: T[K], y: T[K]) => boolean): MonoTypeOperatorFunction<T>;
diff --git a/spec/operators/distinctUntilChanged-spec.ts b/spec/operators/distinctUntilChanged-spec.ts
index 2b22d7e2ba..f90c191ecc 100644
--- a/spec/operators/distinctUntilChanged-spec.ts
+++ b/spec/operators/distinctUntilChanged-spec.ts
@@ -1,7 +1,7 @@
 /** @prettier */
 import { expect } from 'chai';
 import { distinctUntilChanged, mergeMap, take } from 'rxjs/operators';
-import { of, Observable } from 'rxjs';
+import { of, Observable, Subject } from 'rxjs';
 import { TestScheduler } from 'rxjs/testing';
 import { observableMatcher } from '../helpers/observableMatcher';
 
@@ -298,4 +298,34 @@ describe('distinctUntilChanged', () => {
 
     expect(sideEffects).to.deep.equal([0, 1, 2]);
   });
+
+  // This test is to cover a corner case where someone might write
+  // synchronous, reentrant code. At the time this test was authored,
+  // the operator was written in such a way that it would allow
+  // the duplicate non-distinct values to be emitted repeatedly.
+  it('should work properly with reentrant streams', () => {
+    const subject = new Subject<number | undefined>();
+    const results: any[] = [];
+    let count = 0;
+
+    subject.pipe(distinctUntilChanged()).subscribe((n) => {
+      results.push(n);
+
+      // Protect against an infinite loop in this test.
+      // That shouldn't happen.
+      if (++count > 2) {
+        throw new Error('this should have only been hit once');
+      }
+
+      // If we reenter with the same value, it should not
+      // emit again.
+      subject.next(1);
+    });
+
+    // Start with 1.
+    subject.next(1);
+
+    // It should only have emitted one value.
+    expect(results).to.deep.equal([1]);
+  });
 });
diff --git a/src/internal/operators/distinctUntilChanged.ts b/src/internal/operators/distinctUntilChanged.ts
index 8f89e2215a..f7fd07b531 100644
--- a/src/internal/operators/distinctUntilChanged.ts
+++ b/src/internal/operators/distinctUntilChanged.ts
@@ -1,78 +1,181 @@
 import { MonoTypeOperatorFunction } from '../types';
+import { identity } from '../util/identity';
 import { operate } from '../util/lift';
 import { OperatorSubscriber } from './OperatorSubscriber';
 
-/* tslint:disable:max-line-length */
-export function distinctUntilChanged<T>(compare?: (x: T, y: T) => boolean): MonoTypeOperatorFunction<T>;
-export function distinctUntilChanged<T, K>(compare: (x: K, y: K) => boolean, keySelector: (x: T) => K): MonoTypeOperatorFunction<T>;
-/* tslint:enable:max-line-length */
-
 /**
- * Returns an Observable that emits all items emitted by the source Observable that are distinct by comparison from the previous item.
- *
- * If a comparator function is provided, then it will be called for each item to test for whether or not that value should be emitted.
- * The comparator function shourld return true if the values are the same, and false if they are different.
+ * Returns a result {@link Observable} that emits all values pushed by the source observable if they
+ * are distinct in comparison to the last value the result observable emitted.
  *
- * If a comparator function is not provided, an equality check is used by default.
+ * 1. It will always emit the first value from the source.
+ * 2. For all subsequent values pushed by the source, they will be compared to the previously emitted values
+ *    using the provided `comparator` or an `===` equality check.
+ * 3. If the value pushed by the source is determined to be unequal by this check, that value is emitted and
+ *    becomes the new "previously emitted value" internally.
  *
  * ## Example
- * A simple example with numbers
+ *
+ * A very basic example with no `comparator`. Note that `1` is emitted more than once,
+ * because it's distinct in comparison to the _previously emitted_ value,
+ * not in comparison to _all other emitted values_.
+ *
  * ```ts
  * import { of } from 'rxjs';
  * import { distinctUntilChanged } from 'rxjs/operators';
  *
- * of(1, 1, 2, 2, 2, 1, 1, 2, 3, 3, 4).pipe(
- *     distinctUntilChanged(),
- *   )
- *   .subscribe(x => console.log(x)); // 1, 2, 1, 2, 3, 4
+ * of(1, 1, 1, 2, 2, 2, 1, 1, 3, 3).pipe(
+ *  distinctUntilChanged()
+ * )
+ * .subscribe(console.log);
+ * // Logs: 1, 2, 1, 3
  * ```
  *
- * An example using a compare function
- * ```typescript
+ * ## Example
+ *
+ * With a `comparator`, you can do custom comparisons. Let's say
+ * you only want to emit a value when all of its components have
+ * changed:
+ *
+ * ```ts
  * import { of } from 'rxjs';
  * import { distinctUntilChanged } from 'rxjs/operators';
  *
- * interface Person {
- *    age: number,
- *    name: string
- * }
- *
- *of(
- *     { age: 4, name: 'Foo'},
- *     { age: 7, name: 'Bar'},
- *     { age: 5, name: 'Foo'},
- *     { age: 6, name: 'Foo'},
- *   ).pipe(
- *     distinctUntilChanged((p: Person, q: Person) => p.name === q.name),
- *   )
- *   .subscribe(x => console.log(x));
- *
- * // displays:
- * // { age: 4, name: 'Foo' }
- * // { age: 7, name: 'Bar' }
- * // { age: 5, name: 'Foo' }
+ * const totallyDifferentBuilds$ = of(
+ *   { engineVersion: '1.1.0', transmissionVersion: '1.2.0' },
+ *   { engineVersion: '1.1.0', transmissionVersion: '1.4.0' },
+ *   { engineVersion: '1.3.0', transmissionVersion: '1.4.0' },
+ *   { engineVersion: '1.3.0', transmissionVersion: '1.5.0' },
+ *   { engineVersion: '2.0.0', transmissionVersion: '1.5.0' }
+ * ).pipe(
+ *   distinctUntilChanged((prev, curr) => {
+ *     return (
+ *       prev.engineVersion === curr.engineVersion ||
+ *       prev.transmissionVersion === curr.transmissionVersion
+ *     );
+ *   })
+ * );
+ *
+ * totallyDifferentBuilds$.subscribe(console.log);
+ *
+ * // Logs:
+ * // {engineVersion: "1.1.0", transmissionVersion: "1.2.0"}
+ * // {engineVersion: "1.3.0", transmissionVersion: "1.4.0"}
+ * // {engineVersion: "2.0.0", transmissionVersion: "1.5.0"}
+ * ```
+ *
+ * ## Example
+ *
+ * You can also provide a custom `comparator` to check that emitted
+ * changes are only in one direction. Let's say you only want to get
+ * the next record temperature:
+ *
+ * ```ts
+ * import { of } from "rxjs";
+ * import { distinctUntilChanged } from "rxjs/operators";
+ *
+ * const temps$ = of(30, 31, 20, 34, 33, 29, 35, 20);
+ *
+ * const recordHighs$ = temps$.pipe(
+ *   distinctUntilChanged((prevHigh, temp) => {
+ *     // If the current temp is less than
+ *     // or the same as the previous record,
+ *     // the record hasn't changed.
+ *     return temp <= prevHigh;
+ *   })
+ * );
+ *
+ * recordHighs$.subscribe(console.log);
+ * // Logs: 30, 31, 34, 35
  * ```
  *
- * @see {@link distinct}
- * @see {@link distinctUntilKeyChanged}
+ * @param comparator A function used to compare the previous and current values for
+ * equality. Defaults to a `===` check.
+ */
+export function distinctUntilChanged<T>(comparator?: (previous: T, current: T) => boolean): MonoTypeOperatorFunction<T>;
+
+/**
+ * Returns a result {@link Observable} that emits all values pushed by the source observable if they
+ * are distinct in comparison to the last value the result observable emitted.
+ *
+ * 1. It will always emit the first value from the source.
+ * 2. The `keySelector` will be run against all values, including the first value.
+ * 3. For all values after the first, the selected key will be compared against the key selected from
+ *    the previously emitted value using the `comparator`.
+ * 4. If the keys are determined to be unequal by this check, the value (not the key), is emitted
+ *    and the selected key from that value is saved for future comparisons against other keys.
+ *
+ * ## Example
+ *
+ * Selecting update events only when the `updatedBy` field shows
+ * the account changed hands...
+ *
+ * ```ts
+ * // A stream of updates to a given account
+ * const accountUpdates$ = of(
+ *   { updatedBy: "blesh", data: [] },
+ *   { updatedBy: "blesh", data: [] },
+ *   { updatedBy: "ncjamieson", data: [] },
+ *   { updatedBy: "ncjamieson", data: [] },
+ *   { updatedBy: "blesh", data: [] }
+ * );
+ *
+ * // We only want the events where it changed hands
+ * const changedHands$ = accountUpdates$.pipe(
+ *   distinctUntilChanged(undefined, update => update.updatedBy)
+ * );
  *
- * @param {function} [compare] Optional comparison function called to test if an item is distinct from the previous item in the source.
- * A return value of true indicates that it is the same, and a return value of false means they are different.
- * @return {Observable} An Observable that emits items from the source Observable with distinct values.
+ * changedHands$.subscribe(console.log);
+ * // Logs:
+ * // {updatedBy: "blesh", data: Array[0]}
+ * // {updatedBy: "ncjamieson", data: Array[0]}
+ * // {updatedBy: "blesh", data: Array[0]}
+ * ```
+ *
+ * @param comparator A function used to compare the previous and current keys for
+ * equality. Defaults to a `===` check.
+ * @param keySelector Used to select a key value to be passed to the `comparator`.
  */
-export function distinctUntilChanged<T, K>(compare?: (a: K, b: K) => boolean, keySelector?: (x: T) => K): MonoTypeOperatorFunction<T> {
-  compare = compare ?? defaultCompare;
+export function distinctUntilChanged<T, K>(
+  comparator: (previous: K, current: K) => boolean,
+  keySelector: (value: T) => K
+): MonoTypeOperatorFunction<T>;
+
+export function distinctUntilChanged<T, K>(
+  comparator?: (previous: K, current: K) => boolean,
+  keySelector: (value: T) => K = identity as (value: T) => K
+): MonoTypeOperatorFunction<T> {
+  // We've been allowing `null` do be passed as the `compare`, so we can't do
+  // a default value for the parameter, because that will only work
+  // for `undefined`.
+  comparator = comparator ?? defaultCompare;
+
   return operate((source, subscriber) => {
-    let prev: any;
+    // The previous key, used to compare against keys selected
+    // from new arrivals to determine "distinctiveness".
+    let previousKey: K;
+    // Whether or not this is the first value we've gotten.
     let first = true;
+
     source.subscribe(
       new OperatorSubscriber(subscriber, (value) => {
-        const key: any = keySelector ? keySelector(value) : value;
-        if (first || !compare!(prev, key)) {
+        // We always call the key selector.
+        const currentKey = keySelector(value);
+
+        // If it's the first value, we always emit it.
+        // Otherwise, we compare this key to the previous key, and
+        // if the comparer returns false, we emit.
+        if (first || !comparator!(previousKey, currentKey)) {
+          // Update our state *before* we emit the value
+          // as emission can be the source of re-entrant code
+          // in functional libraries like this. We only really
+          // need to do this if it's the first value, or if the
+          // key we're tracking in previous needs to change.
+          first = false;
+          previousKey = currentKey;
+
+          // Emit the value!
           subscriber.next(value);
         }
-        prev = key;
-        first = false;
       })
     );
   });