Diop has signed a five-year contract at Craven Cottage after spending four seasons at the London Stadium.
The Frenchman had one year remaining on his contract with the Hammers but will now join their London rivals after losing his spot in David Moyes' starting eleven.
Diop made 119 appearances in total for West Ham but played just 13 times in the Premier League last season after Kurt Zouma joined from Chelsea.
Speaking to FFCtv, Diop said: "I'm very happy, very happy to join Fulham. I hope it's going to be a very good chapter of my career.
"It's all official now, so I'm happy. I just want to train with the team now, and let's get to work."
Fulham vice-chairman Tony Khan added: "I'm very excited that we've brought Issa Diop to Fulham.
"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."
The 25-year-old is Fulham's seventh summer signing after Joao Palhinha, Andreas Pereira, Kevin Mbabu, Bernd Leno, Manor Salomon and Shane Duffy.
Fulham started their 2022/2023 season with a creditable 2-2 draw against Liverpool last Saturday.
They play at Wolverhampton Wanderers this weekend.
