Credit: Brian Prahl/Splash News Online

Looks like Kourtney Kardashian's baby weight is a thing of the past!

The E! reality star flaunted her post-baby bod in a skimpy bikini while hitting the surf in Miami Beach, Fla. on Saturday. (Sister Kim was also on hand for the summer fun.)

Kourtney, 31, welcomed her son Mason -- her first child -- last December. (Mason's dad is her boyfriend Scott Disick).

Even Kourtney's little sister is envious of her slimdown. "My sister weighs less than I do, so that's kind of fascinating to me!" Kim, 29, told last April. "She looks amazing."

How did Kourtney do it, then? "Definitely breastfeeding helps," she told Us. "Everyone says that, but it's really true -- and it's just being a busy mom."

She added that motherhood makes her feel more -- not less -- sexy.

The E! reality star has said that she has felt sexier since becoming a mom. "It brings out a whole new glow or something," she told Us.