We've confirmed the celeb couple is listing their kick-ass 4,000 sq. ft. Park Ave pad for a cool $17.5 million, as first reported by the NY Post. You'll recall ... J Lo and A-Rod bought the 3 bedroom, 4.5 bathroom apartment back in March, dropping $15.3 million on their first real estate purchase together.

As for why they're already unloading this primo location -- in the world's tallest residential building, btw -- think family planning. We're told the couple's on the hunt for something a little more modern family friendly ... remember they have 4 kids between them.