Nine days after his win at Doncaster, Muhtarram was moved up in class to contest the Irish Champion Stakes at Leopardstown.
Ridden by the veteran Willie Carson he took the lead inside the final furlong and won by half a length from the favourite Opera House.
As a five-year-old, Muhtarram began his season by travelling to Italy, where he won the Premio Presidente della Repubblica in Rome on 15 May.
He returned to England in June for the Prince of Wales's Stakes at Royal Ascot where he defeated Ezzoud by a neck.
He won the race for the second time, defeating Eltish by a short-head with the odds-on favourite Balanchine in fifth place.