The 20-year-old Australian initially came to The County Ground on loan last season and, after re-joining on loan in the summer, has put pen to paper on a permanent contract.
"I'm very pleased to have signed," he told the club's official website. "It's a good bunch of lads and I'm happy to be here.
"I hope to keep doing what I'm doing and doing well."
Luongo has made six appearances so far this season and scored his second of the season in the emphatic 5-0 win over Crewe on Saturday.