The singer got swept up in Madge's "Vogue" segment during her Miami "Celebration" tour stop Sunday, where Ricky was the featured celeb guest and judging the dancers next to the main talent ... and at one point during the sexy dance, Ricky appears to have gotten aroused.

The reason we believe that to be the case -- Ricky looked erect while seated, and even when he stood up ... the guy seemed to be saluting from the waist below.

Makes sense, we suppose, when you see what these dancers were doing on Ricky for a quick minute -- including rubbing up against his junk, something he appears to have relished.

In the moment, he also didn't really acknowledge the fact he was erect either -- other than continuing on with his whole performance, that is, until the routine was over and done with.

Madonna didn't seem to notice herself, and the show went on like normal. But many in the audience certainly saw it ... and they're posting about it on social media. Strangely enough, this footage of Rick with what seems like a boner comes from his own account.

So, if he was feeling good ... he definitely ain't trying to hide it.