The 26-year-old Moroccan international had been playing for two seasons in Rennes, who had bought him from Dijon for nearly 5 million euros and will realize their greatest added value.
His contract was extended last summer until 2025.
"I was really excited when I heard of the interest from West Ham United," Aguerd said in the West Ham statement.
"When I heard about it, I knew I had to go to the Premier League, because it's every player's dream," added the left-hander.
Aguerd was on the shelves of David Moyes, the general manager of the Hammers.
"We've been following his progress for some time and I'm very happy that we were able to sign him. It's a nice reinforcement that will add to our defensive options," he said.
West Ham finished the season in 7th place in the English league.
Aguerd, pillar of the central hinge of Stade Rennais and author of four goals last season in L1, announced Saturday on social networks his departure from the Breton club.
With him on the field, Rennes scored an average of 1.9 points per game last season in Ligue 1, against 0.9 points for the seven games in which he was at the African Cup of Nations, suspended or sick.
His transfer will leave a gaping hole in the central defense.
"We will have to recruit two central defenders", had slipped the shareholder François Pinault to the press, evoking his possible departure a few weeks ago.
© 2022 AFP