Adeyemi joined the Bees in August and has played 21 games during the first half of the season, scoring against Southend in the last round of the FA Cup.
The 21-year-old midfielder was set to see his initial loan expire this week but a deal has been agreed for him to stay at Griffin Park to help Uwe Rosler's team maintain their League One promotion push.
"It is fantastic that Tom has signed until the end of the season," said Brentford boss Rosler. "He is a very important part of our squad and he has shown his quality this season. He is a very good professional and always strives to be better.
"Tom is the right player for us at the right time; he gives us something different to what we have in central midfield.
"I am looking forward to working with Tom and I hope he will continue to have a strong level of performance for the rest of the season."
The Bees' sporting director Mark Warburton added: "We have been very pleased with the impact Tom has made and the way he has applied himself both on and off the pitch from the outset.
"We are delighted that Tom and Norwich City have agreed to extend the loan for the rest of the season.
"Everyone at Brentford is looking forward to seeing Tom help us continue our progress for the rest of the season."