The back-row forward, a product of the Chiefs' academy, has spent time on a dual registration deal at Championship side Cornish Pirates this year.
Simmonds, 22, will make his first Premiership appearance for Exeter against Wasps on Sunday.
"He has shown an awful lot of very, very good qualities," said head coach Rob Baxter.
Exeter Chiefs forward Sam Simmonds has signed a new two-year deal with the Premiership club.