The centre-back downed tools over the weekend and missed West Ham's 2-0 defeat to Manchester City, forcing teammate Ben Johnson to play out of position in the heart of the defence against Erling Haaland.
Diop's decision to step aside made him instantly unpopular with the West Ham faithful, with many saying that his attitude was not welcome in the dressing room. He didn't last much longer, and will no doubt be booed when he returns with his new club in early October.
The 25-year-old says he is ready to start a new chapter of his career after signing a five-year contract in the move across London.
Diop made 121 appearances for the Hammers during his four-year stint at the club and was a key figure in their run to the Europa League semi-finals last season.
"I'm very happy, very happy to join Fulham, I hope it's going to be a very good chapter of my career," Diop told the Fulham website.
"It's all official now, so I'm happy. I just want to train with the team now, and let's get to work."
Diop, who has represented France at youth level, moved to the Hammers from his boyhood club Toulouse in the summer of 2018.
"I'm very excited that we've brought Issa Diop to Fulham," vice-chairman Tony Khan said.
"Issa is a talented and imposing defender whom we've pursued for a long time, and his experience and formidable presence will be important to our squad in this Premier League season and for us to build a bright future in the years ahead."