The 21-year old midfielder joined the Bees from Norwich City in August and was set to return to Carrow Road this week, however, both clubs have agreed to prolong the stay.
"It is fantastic that Tom has signed until the end of the season," said Brentford boss Uwe 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."
Adeyemi has made a total of 21 appearances for Brentford, scoring once in the third round FA Cup draw with Southend United.