Boxing Star Andy Ruiz Gets Massive Backside Tattoo, Butt Cheeks Included! Andy Ruiz Boxer Gets Massive Backside Tattoo ... Butt Cheeks Included!!! 1.3K 10/5/2021 7:35 AM PT

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.

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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."

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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.