Mac McClung's vert may be just a smidge less next dunk contest ... 'cause the high-flying hooper just got his whole arm tatted in NYC!

TMZ Sports has learned the 25-year-old guard hit up tattoo artist Michelle Santana out in SoHo after peeping some of her work on social media.

We're told McClung wanted an arm sleeve with sentimental meaning ... so the two got together at Bang Bang Tattoo and created a showpiece.

The sleeve on his right arm features a lot of intricate work ... there's the ocean, a butterfly, birds, and a kid reading a book under a tree which Santana says represents McClung keeping his inner child's hopes and dreams alive.

The piece also features a mountain ... which we're told signifies where McClung came from in Virginia.

Mac also repped his high school, Gate City, in Scott County, VA.

"He had the design, everything very clear in his head so he decided where each element would be placed in which area of his arm," Santana said.

The arm tattoo took just two sessions. Santana -- who's worked on Ella Mai, Becky G, and Kelsea Ballerini -- told us they plan to add more to the pieces.

If Mac -- who played college ball at Georgetown and Texas Tech -- doesn't share pics before, the work will be front and center on the court when McClung hoops this upcoming season, whether it's in the NBA or G-League.

Of course, McClung's known for his insane hops and dunking ability ... winning his 2nd Slam Dunk Contest during NBA All-Star weekend in February.