"J.Lo" received mixed reviews from contemporary music critics. At Metacritic, which assigns a normalized rating out of 100 to reviews from mainstream critics, the album received an average score of 52 based on 11 reviews. Michael Padletta was positive in his review for "Billboard", praising Lopez for "assembling a sophomore set that's brimming with irresistibly hip-hop spiked songs [...] The result is a set with hip, decidedly time-sensitive feel, which goes a long way toward giving Lopez the credibility she'll need to develop a long-term music career." He also praised Lopez's voice, writing that it "sounds more confident, as she takes more chances with a voice that is technically limited but well-suited to the material she handles". Amy Linden of "Sonic.net" particularly praised the album for being "a feisty, damn-I-know-I'm-all-that attitude, combined with pulsating, insistent beats that leap out of the speakers and make you wanna move". AllMusic's Stephen Thomas Erlewine gave the album a rating of three-out-of-five-stars, describing it as "essentially the same album" as "On the 6" and was unfavorable of its running length. However, he noted, "it has just about the same number of strong songs, all of which sounding of a piece with 'On the ', which makes it a success on a certain level."