Bees boss Uwe Rosler told the club's official website: "Tom has been on the cards for a long time and was the player I wanted to bring in from day one.
"He's a dynamic midfielder, who adds to us very good ability, is good on the ball, powerful and adds height and an enormous amount of work ethic."
Adeyemi made 45 appearances for Oldham Athletic during a loan spell last season.
The 20-year-old becomes Brentford's second loan signing of the day, after teenage left-back Jake Bidwell arrived from Everton.
