Balcombe made his Bees debut against Newport County in August, saving two spot-kicks during a penalty shootout victory in the second round of the Carabao Cup.
He signed a new four-year contract with Brentford in October.
Advertisement
Advertisement
St Mirren finished fifth in the Scottish Premiership last season. It means they will be in the Europa Conference League second qualifying round ahead of next season.
Balcombe is a former Brentford B player.