Boxing Star Andy Ruiz Gets Massive Backside Tattoo, Butt Cheeks Included!

When Andy Ruiz gets a full backside tattoo, the boxing star goes all in -- neck, back ... and even butt cheeks!

The former heavyweight champ showed off new back ink Monday ... and it's HUGE -- stemming from the top of his neck to the bottom of his bum.

Ruiz got the piece done by Ish Tattoos ... and it's some seriously impressive work -- especially considering Andy's 6 feet 2 and nearly 300 pounds.

The tat artist documented some of the work on his social media page ... saying it took two full sessions to complete.

Ruiz got a big warrior with wings inked under his previous "Victorious" tat ... and he also added a neckpiece with the bible verse, "Corinthians 15-57."

Andy was clearly happy with the results ... captioning a photo of the artwork, "Back and cheeks done hahaha. Thanks to my boy Ish Tattoos."

Unclear when we'll see Ruiz show off the new ink next in the ring ... he's coming off a big May victory over Chris Arreola -- and is reportedly gunnin' for Luis Ortiz or Charles Martin next.