In 2021, it was announced that Biffle would pilot the No. 69 car in select 2021 Camping World SRX Series races. He started his season off at the inaugural SRX race at Stafford Motor Speedway, winning the first heat but getting caught up in a wreck in the second heat. For the main event, he switched to a backup car and drove to second by the end of the race, behind only Doug Coby.