The 24-year-old singer got a friend to approach the 19-year-old, who plays Holly Cunningham in the Channel 4 soap, during the after-party, the Daily Star Sunday reports.
"JJ couldn't keep his eyes off Wallis. She's tall and has a gorgeous smile," a source said.
"As she was dancing, he was totally captivated by her. His band are about to take the world by storm and you'd think he'd have the confidence to chat up a pretty young actress - but he was totally in awe of her.
"Wallis is a friendly girl and was happy to go over and meet him after his friend put in a good word for him. They started chatting and the sexual chemistry between them was instant.
"They were talking and flirting for ages and realised they had so much in common. Before JJ left they swapped numbers and the pair have been in contact ever since.
"JJ is desperate to take her out on a date and they're currently looking at when they can hook up."
Union J release their debut single 'Carry You' next Sunday (June 2). Watch the video below: