The left-back, who represented his country at the 2026 FIFA World Cup, joins the Bees for a 40 million pounds transfer after West Ham were relegated from England's top-flight last season.
"I'm really happy, really happy to be here at this amazing club," said the 21-year-old. "I was looking forward to it. Now I'm here and I'm ready to give everything.
"They have a really good connection with the players. I think that's really important when you're a player, to have a good connection with your fans.
"They can help you a lot and give you a lot of confidence as a player. I want that and I'm really excited to meet them."
"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, impressing the full-back position.
