The festival takes place at Blackpool Pleasure Beach Arena on Saturday to Monday, October 21 to 23.
Joining the previously announced acts including Pixie lott, The Vamps and X Factor 2016 winner Matt Terry, will be 5 After Midnight - who also appeared on the talent show last year.
The trio will be appearing at all six shows across the three-day festival.
Singers Kieran Alleyne, Jordan Lee and Nathan Lewis signed to Simon Cowell’s Syco label after appearing on the show, and released their first single Up In Here.
Their now working on their debut album to be released in 2018, and will appear alongside Diversity and JoJo Siwa at all the shows.
Kieran said: “Slimefest is going to be so much fun, we’re so excited to be performing in Blackpool.
“Maybe we’ll get slimy in the Slime Pit too... If we’re lucky.”
Also appearing across the weekend will be New Hope Club and M.O, with Diversity’s Jordan Banjo and Perri Keily hosting, plus the stars of Nickelodeon’s School of Rock Breanna Yde and Ricardo Hurtado.
Coun Gillian Campbell, deputy leader of Blackpool Council, said: “It’s great to hear the news that 5 After Midnight will be joining the rest of the Slimefest talent at this year’s shows.
“Tickets are selling fast now, so would encourage everyone to book early for what promises to be the best Slimefest to date.”
Ticket options include seated or standing arena tickets and Slime Pit tickets offered exclusively to kids and guaranteeing an uniquely slimy experience close to the stars.
There is now only limited availability to all six shows.
Full details on available tickets, online at www.nick.co.uk/slimefest. Tickets can be booked by telephone only on 0871 222 9090.
Last year’s Slimefest was attended by 4,000 people at the Empress Ballroom, running alongside the Switch-On celebrations, with Diversity, Conor Maynard, Alesha Dixon and Aston Merrygold on stage.