The 34-year-old scored seven goals and added one assist in 35 appearances for the Hammers in 2025/26, 12 of them starts, and now moves to west London as a free agent to provide competition and cover for in-form striker Igor Thiago, who scored 22 Premier League goals last season.
Manager Keith Andrews explained the thinking behind the move: "Callum is someone that everyone will be familiar with; he has been a Premier League goalscorer for such a long time.
"He gives us more options in that area of the pitch, and will help Igor Thiago and Kaye Furo with the experience that he has.
"He's just got that nous to score goals, as well as that experience around the training ground.
"This is another really good signing for us."
For his part, Wilson, who sits five goals short of a century in the Premier League, says he sees the move as a chance to add to Brentford's attacking options rather than compete head-to-head with Thiago.
"It's nice, it's amazing, I'm grateful for the opportunity," said Wilson. "I'm delighted to be here and joining as part of the family.
"I spoke with the manager. We had a good conversation and he was very enthusiastic. He told me about his ambition for the club and what his ambitions and desires are, and I felt like I could add quality to that."
He added: "Many people in my position as a striker, off the back of Thiago having a fantastic season, would probably shy away from the challenge of joining the club. For me, it's about coming and sharing the load, helping out as a striking department and also adding my qualities as well.
"We're different strikers, but sometimes I'm guessing we'll play together as well, so it'll be nice to be part of it and help the team climb up the league. The club were so close to Europe last year and, this year, hopefully, we can get over the line."
Wilson will link up with several familiar faces at the Gtech Community Stadium, including Jordan Henderson and fellow new arrival Jaidon Anthony, who signed from Burnley earlier this summer and previously played alongside Wilson at Bournemouth.
"I spoke to Jordan Henderson as well. He sung the praises of the club. Jaidon Anthony just joined as well and I played with him at Bournemouth. There's some familiar faces there," Wilson said.
"There is unbelievable quality in the dressing room, and I've met people today who are all very welcoming. You do get that family feel here so, like I say, I'm looking forward to getting started."