John Cena's going for Round 2 against a thinning hairline ... and this time he's hoping to take home the belt. The WWE star went bald and beautiful for his next treatment and proudly posted the pic online.

The retired wrestler shared a shot of himself rocking a smooth head as he shakes hands with hair restoration surgeon Dr. Ken Anderson.

As you know, John's already undergone a hair transplant with Anderson Center for Hair ... and now he's back in the ring hoping to finally defeat balding.

He wrote on Facebook that "went all in" by shaving his head this time around, saying he wanted to make sure he got the "best possible results."

We caught up with John back in April during WrestleMania 42, where he told us all about what he looked like post-op.

He explained that his head had swelled during the healing process, making him look like he had "an alien head."

And hey, if this transplant doesn't work out ... now we know John still looks amazing bald.