The boyband confirmed that Richardson will return for the recording of their eighth studio album later this year.
Brian Littrell, Nick Carter, AJ McLean and Howie Dorough made the revelation while performing at London's O2 Arena this weekend.
The 40-year-old, Littrel's cousin, left the group in 2006 to pursue other avenues but was not replaced.
Richardson has previously reunited with the Backstreet Boys for live performances.
AJ McLean announced on YouTube yesterday that he and wife Rochelle De Anna are expecting a baby.
Watch the Backstreet Boys' music video for classic 'I Want It That Way' below: