Nigel Lythgoe won't participate in this year's edition of "So You Think You Can Dance" ... making the decision after claims of sexual assault were leveled against him by Paula Abdul.

Nigel tells TMZ, "I have informed the producers of ‘So You Think You Can Dance’ of my decision to step back from participating in this year’s series. I did so with a heavy heart but entirely voluntarily because this great program has always been about dance and dancers, and that’s where its focus needs to remain."

Nigel adds, "In the meantime, I am dedicating myself to clearing my name and restoring my reputation."

TMZ broke the story ... Abdul filed a lawsuit against Nigel last week claiming she'd been sexually assaulted by him on at least two occasions -- once when she was a judge on "American Idol" and again while she worked on 'SYTYCD.'

During the 'Idol' incident, she says Nigel grabbed her breasts and genitals while trying to kiss her in an elevator. Paula claims Nigel tried to force himself on her during "SYTYCD."

For his part, Nigel has denied Paula's allegations, calling them, "false and deeply offensive."