The Australian international has agreed a three-year deal with the London side.
Luongo initially joined Swindon on loan in 2013 before completing a permanent move to Wiltshire later that summer. He made over a century of appearances for the Town and netted 14 goals.
The midfielder would also make his Australian international bow while at The County Ground and became the first Town player for over 20 years to represent his country at a World Cup when he was included in the Socceroos' squad for the 2014 finals.
Everyone at Swindon Town would like to thank Massimo for his service to the club and wish him all the best in his future endeavours.