The Morocco international is the Hammers' first signing of the summer.
Aguerd has established himself as one of the best young defenders over the last four seasons, having moved from Dijon to Rennes in the summer of 2020.
Aguerd made 40 appearances across all competitions last season, scoring four goals as Rennes finished fourth in Ligue 1.
The 26-year-old has signed a five-year contract at the London Stadium.
"I was really excited when I heard about West Ham United's interest," said Aguerd. "When I heard about it, I knew I needed to go to the Premier League, because it's a dream for every player.
"West Ham is a historic club. I saw a few of their games this year, and I saw the fans and the atmosphere were fantastic, with the bubbles! I talked with the manager and he showed me that he was very interested, so it was easy to choose to come to West Ham."
West Ham boss David Moyes added: "We have tracked his progress for some time and I'm very pleased that we have been able to complete the signing. He's a great addition that will add to our defensive options.
"I have been really impressed with Nayef's character and attitude during our discussions. He has a great desire to improve and be successful, and I am sure he will fit in very well to the environment we have at Rush Green."
Aguerd has won 25 caps for Morocco and is set to play at the FIFA World Cup in Qatar later this year.
