The Warriors have announced that the 28-year-old tighthead prop will remain at Scotstoun until May 2016.
Former Leeds front-row Cusack has made 33 appearances since arriving in Glasgow in June 2011 from Doncaster.
He said: "This was an easy decision for me to make. We're building an exceptionally strong side.
"I want to help this club achieve success and that means committing to the direction the club are going."
Glasgow head coach Gregor Townsend added: "He is developing a formidable reputation as one of the best tightheads in the game and to have him here at Scotstoun for the long term is terrific news."