Brentford said the 21-year-old, who went to the 2026 World Cup with Senegal, had signed a five-year deal with an option of a further year on the club's side.
"He's a player that fits our ideal age profile, with lots of room to still develop," Brentford manager Keith Andrews told the club website.
"I think he had a really good year at West Ham in his first season in English football, and some of the qualities and attributes he has as a full-back really add to an already impressive part of our team."
Diouf joined West Ham last year from Slavia Prague and became a regular for the side who were relegated at the end of the season. He is reunited at Brentford with former Hammers teammate and striker Callum Wilson, who moved from East to West London last month.
"His left foot and his delivery are unbelievable, so I think we've got someone who's hard-working, who's ready to fight for the team, and it's great competition for everyone in the squad," said Wilson.
