The exes posted respective tributes to their pit bull mix, Lua, on Saturday ... announcing their longtime pet had passed on. Tom threw up a photo of his two children with Gisele -- Benjamin and Vivian -- hovering over their doggy, which had its eyes closed.

He wrote, "We love you Lua RIP." He posted another photo of Lulu, and added ... "Forever in our hearts." In yet another IG Story slide, he uploaded a boomerang of his daughter playing with Lulu.

Gisele, likewise, threw up a bunch of shots of the canine in a sprawling IG post ... and she had a sweet shout-out to Lulu that touched on just how much she meant to the family.

She wrote, "Our little Lulu, our guardian angel is gone to heaven. She will forever live in our hearts. We already miss her sooooo much! #unconditionallove RIPLua." Her caption was also translated to Portuguese ... and she included a ton of awesome photos of their doggo.

Unclear when exactly they got Lua, but we know they adopted the pit bull at some point in their marriage ... and that their kids grew up with it too. They have another dog too, Fluffy.

All dogs go to heaven.