The New Approach colt became the first horse to carry the royal blue silks of Godolphin to victory in the the premier Classic at Epsom last season when prevailing under Northern Racing College graduate William Buick.
But a leg injury kept him off the track for the remainder of the campaign.
Masar made his long-awaited return in the Hardwicke Stakes at Royal Ascot last month – and performed with credit to finish fifth after stumbling soon after the stalls opened.
However, he disappointed when a hot favourite for last week’s Princess of Wales’s Stakes at Newmarket – trailing home last of six runners behind the Mark Johnston-trained Communique – and connections have now decided to call time on his racing career ahead of a career as a stallion.
“After deep discussions with His Highness Sheikh Mohammed, the decision has been made to retire Masar,” said Appleby.
“He’s a horse who has given us many great days, including a day at Epsom last June that none of us will ever forget.
“From a personal point of view, he was my first Classic winner – and while it will be sad to see him leave the yard, we have to do what is the right thing for the horse, and I’m pleased he’s retiring from racing fit and well.”
Meanwhile, Dream Of Dreams may try for better luck in the Group One Sprint Cup at Haydock after finding conditions unsuitable in the July Cup at Newmarket.
Connections reported the ground was too fast on Saturday as the five-year-old – the mount of Thirsk jockey Danny Tudhope – ran below expectations and finished 10th behind behind Aidan O’Brien’s impressive winner Ten Sovereigns.
The Sir Michael Stoute-trained colt went to Newmarket as the number one hope among the older horses, after running Blue Point to a head in the Diamond Jubilee Stakes at Royal Ascot.
But Bruce Raymond, racing manager for owner Saeed Suhail, said: “It was too firm for him, and he was probably unsuited by the course.”
Caspian Prince is set to cross the Irish Sea to run for the third successive year in the Sapphire Stakes at the Curragh.
The veteran sprinter lifted Sunday’s Group Two prize in 2017 and was second to Havana Grey 12 months ago.
The 10-year-old is being kept busy by trainer Mick Appleby, after being beaten just a length when fourth to Copper Knight in the City Walls Stakes at York on Saturday.
A strong British challenge also includes Malton trainer John Quinn’s El Astronaute, who is chasing a third successive victory in Ireland following June wins at Cork and in the Rockingham Stakes over this course and distance. Yorkshire could also be represented by Richard Fahey’s Mr Lupton.