Simpson joined Huddersfield last season but struggled to get into their first team on a regular basis as he had with the Sky Blues.
Huddersfield Boss Lee Clark does not see Simpson in his plans and has loaned him to Brentford for the season.
Speaking about signing Simpson, Brentford boss Andy Scott said to his club's website:"I've been aware of Robbie ever since he played for Cambridge.
"I am sure he will replicate the form he showed when earning himself a move to the Championship with Coventry."