Lopez, who was seen wearing a glittery red dress with a thigh-high slit, entered into the dancing stage first and got down to "Three Times a Lady" by the Commodores.
Lopez was deemed the winner after dancing to "MacArthur Park" by Richard Harris following Fallon's dance number to "Babe" by Styx.
"You're clearly the best dancer," Fallon said about Lopez's performance. "That was amazing what you just did to 'MacArthur Park.'"
The singer and late night host then fast danced together to "Reunited" by Peaches & Herb.
Lopez attended the 2018 Met Gala on Monday with her boyfriend, former baseball star Alex Rodriguez.
"I describe it as the luckiest day of my life," Rodriguez said recently on Ellen about the first time he met Lopez.