Sources close to the show tell TMZ ... Leslie is NOT returning for season 45 of 'SNL' and it's all her decision.

Fact is ... Leslie's got a lot more on her plate now, thanks in no small part to her time as one of the biggest stars on 'SNL' -- where her personality, wit and skits made her a hot commodity in Hollywood.

As we've reported ... Leslie's already filming the 'Coming to America' sequel with Eddie Murphy ... and even though he's going to host 'SNL' this season for the first time in 35 years, she won't be there to enjoy it as a cast member.

Also on deck for Leslie ... she's signed on with Netflix for a comedy special, and we're told she's got some more movie projects in the works.

It's the end of the road for Leslie and 'SNL' ... and she had an amazing run over 5 seasons ... but sometimes, the grass just looks greener elsewhere.