The 29-year-old centre-back has signed a four-year contract running to the summer of 2030 for an undisclosed fee, arriving in Suffolk fresh off the back of a World Cup campaign in which he started five of Morocco's six games, including a stoppage-time equaliser against the Netherlands in the last 32, as the Atlas Lions reached the quarter-finals before losing to France. Reports elsewhere have put the fee in the region of £8.5m.
Diop brings significant Premier League pedigree to Portman Road, having made 173 top-flight appearances in England, the majority of them during a four-year spell at West Ham before he moved to Fulham in the summer of 2022, where he went on to make 96 appearances in all competitions.
He switched international allegiance from France to Morocco under FIFA eligibility rules earlier this year.
"I'm excited to take the next step in my career at Ipswich Town," Diop said. "I know how big the club is and have enjoyed all of the conversations I have had before making the move. I know this is the right place for me.
"It has been a busy summer for me, representing Morocco at the World Cup, but I am proud to be joining Ipswich Town and am looking forward to meeting the staff, my team-mates and the supporters very soon."
Ipswich Town manager Gary O'Neil said: "Issa is an experienced player who has played a significant amount of his career in the Premier League, so we're delighted to have him with us. He is an imposing defender with good quality and a will to win, making him an excellent addition to the group. Having played and scored at the World Cup this summer, we're looking forward to Issa joining up with the squad soon."
Diop is Ipswich's sixth summer signing and will join up with the rest of the squad once they return from their pre-season training camp in La Manga, Spain.
He follows Kayne van Oevelen, Abdul Fatawu and Emersonn through the door this summer, while loan spells for Cedric Kipre and Chuba Akpom have also been converted into permanent deals as O'Neil looks to build a squad capable of a longer stay in the Premier League than the club managed following their promotion in 2024.