On May 20, Cotto started working with Golden Boy Promotions to finalize a deal against Yoshihiro Kamegai (27-3-2, 24 KOs). Miguel Cotto Promotions Executive Hector Soto said a deal should be finalized in the coming week. On May 24, it was announced a deal was made for a fight between Cotto and Kamegai to take place on August 26, 2017, at the StubHub Center in Carson, California live on HBO. This would be Cotto's 23rd appearance on the network. It was confirmed that vacant WBO light middleweight championship would be at stake, the title was vacated by Canelo Álvarez, who moved up to middleweight. A day after signing a deal with Golden Boy, Cotto revealed that he would fight again on December 2, 2017. Cotto weighed 153.6 pounds for the fight with Kamegai coming in slightly heavier at 153.8 pounds. Cotto reiterated that he would retire on December 31, 2017. In front of 7,689 st the StubHub, Cotto defeated Kamegai via unanimous decision after 12 rounds to claim the vacant WBO light middleweight title and became a 6-time world champion. The three judges scored the fight 120–108, 119–109, and 118–110 all in favour of Cotto. Kamegai gave a good effort but fell short every time he would connect with a punch, Cotto would counter with a combination of his own. Cotto spent the early rounds backed against the ropes on the back foot and still managed to do some damage landing hooks to the head of Kamegai. After round 10, Cotto started boxing on the outside and decided to settle for a decision win, he explained in the post-fight, "I think I did my best and I'm happy with my performance. (Kamegai) is a tough fighter. By the fifth or sixth round, I knew I was not going to be able to stop him." Cotto confirmed he would fight again in December, preferably the winner of Canelo-GGG. The fight, which took place on HBO averaged 730,000 viewers and peaked 805,000 viewers. The fight ended before the Floyd Mayweather Jr. vs. Connor McGregor fight started on Showtime PPV.