Jimmy Fallon proved he's a comedic genius with a hilarious performance of "(It's Not My) Baby" ... poking fun at Justin Bieber's reported baby mama drama.

Rocking a leather jacket and Bieber style 'do, Fallon was joined on stage by The Roots.

As for Bieber, himself, he's been slapped with a paternity lawsuit involving 20-year-old Mariah Yeater who claims the 17-year-old heartthrob fathered her son.

Yeater's legal team released the following statement:

    "Mariah Yeater is pursuing a modest and rightful claim. There is credible evidence that Justin Bieber is in fact the father of her baby. The evidence will be revealed in court."


The lawyers add, "She is not seeking exorbitant compensation. She merely requests that Justin Bieber provide a DNA sample to verify that he is the father of Mariah Yeater's baby, and if so, to help provide child support as required by California law."