The 10-year-old became the first horse since Best Mate to win back-to-back Gold Cups at Cheltenham in 2019 and 2020, but only finished third behind Minella Indo when going for the hat-trick.
Al Boum Photo's racing career had been on the wane in 2022. Despite a win at Tramore for the fourth successive year on New Year's Day, he struggled at Cheltenham and Punchestown before being pulled up in the Grand Steeple-Chase de Paris at Auteuil last time out.
Mullins tweeted: "Al Boum Photo and Melon retired today. Fantastic horses for Marie and Joe Donnelly and our yard. Al Boum Photo gave us two Cheltenham Gold Cups which was very special and Melon was such a consistent performer. We wish them all the best in their new eventing careers."
Joining him in retirement is stablemate Melon, who had a rather less successful time at Cheltenham.
He finished runner-up at four successive Festivals, chasing home Labaik in the 2017 Supreme, Buveur D'Air and Espoir D'Allen in the following two Champion Hurdles and Samcro in the 2020 Marsh Novices' Chase.
Melon, also 10 and like Al Boum Photo owned by Joe and Marie Donnelly, finished second in a Grade Three at Killarney just last month.