Benson signed from Charlton in January 2012 and made an immediate impact as Swindon won the League Two title under Paolo Di Canio. He scored 12 goals, including the winner as the Robins dumped Wigan out of the FA Cup.
The 33-year-old fell out of favour and spent much of last term out on loan at Portsmouth and Cheltenham. He last featured for Swindon against Aston Villa in the Capital One Cup in October 2012.
Benson will look to revive his career at Kenilworth Road during his season-long loan and prove he is worth a contract.