The boyband join Professor Green, Soul II Soul and Billy Ocean in the line-up for the two-day event at Welford Road in the summer. Aswad, The Real Thing and Stereo Nation are also on the bill with more acts still to be announced.
Union J were formed in last year’s X Factor, released their debut album, Union J, in October and have enjoyed a couple of top ten hits.
Singer JJ Hamblett said: "Our last visit to Leicester for our book-signing was a bit crazy, but we loved it and we're really looking forward to coming back in July.
"It will be the first time we've performed in Leicester so we want to put on a brilliant show."
The Leicester Music Festival will showcase 16 acts across two days, spanning soul, urban, rock, reggae, pop and R’n’B at Welford Road Stadium, home of the Leicester Tigers, from noon until 10.30pm on Friday, July 25 and Saturday, July 26, 2014.
A limited release of early bird tickets, priced £37.50, are available from www.theticketfactory.com