Kevin Clash -- known as the voice of Elmo -- is facing NEW allegations that he had a sexual relationship with a SECOND underage boy ... TMZ has learned.
The accuser -- who is now in his 30s -- has filed a lawsuit against Clash ... claiming the two met on a gay phone chat line back in 1993 ... when the accuser was 15 years old and Clash was 32.
Among the allegations, Singleton claims, "[Clash] trolled gay telephone chat line rooms to meet and have sex with underage boys."
In the lawsuit, filed in federal court in NYC, Singleton claims Clash "groomed [the accuser] to gain his trust by, among other things, taking him to nice dinners and giving him money."
Singleton claims while Clash was featured on "Sesame Street," he was "preying on teenage boys to satisfy his depraved sexual interests."
In the suit, Singleton claims ... although the sex occurred nearly 20 years ago, he didn't take action until now because ... he "did not become aware that he had suffered adverse psychological and emotional effects from Kevin Clash's sexual acts and conduct until 2012."
Singleton is demanding more than $5 million.
We broke the story, Clash struck a settlement with a previous accuser who initially claimed he had a sexual relationship with Kevin beginning when he was 16 years old. As part of the settlement, the accuser issued a statement recanting his claims.