J Lo's high school sweetheart, David Cruz, died Saturday in Mount Sinai West Hospital in Manhattan ... this according to the NYC Medical Examiner's Office. We're told David died of heart disease -- no other info was immediately available.

Jennifer and David got together when she was just 15 (he's only a year older than her, BTW) and they dated for about 10 years before calling it quits in the mid-'90s as J Lo's fame was skyrocketing.

David and Jennifer did attend a few events together back in the day -- when she wasn't well known -- they sure looked like a happy couple.

David's partner of 18 years, Isa, tells us, "He was kind and loving. He never held on to anything and was always very open. He was a devoted father, he helped raise his step-son who is now in the Marines. He loved the Yankees and Knicks. He loved going to the theatre with me. My favorite moment was family date night, because it wasn't just special for me but for the kids as well. He always made sure to end things with an 'I love you.'"

Jennifer's gone on to have super high profile relationships since then -- including dating Diddy, Ben Affleck, Casper Smart, Drake, Marc Anthony (whom she married) and now ... Alex Rodriguez, to whom she's currently engaged.

David was 51.