Nick Cannon -- No Wedding Ring ... on America's Got Talent

It seems Nick Cannon is making his break-up with Mariah Carey official ... 'cause he appeared on "America's Got Talent" Tuesday night sans wedding ring.

Cannon was hosting a live episode of the show and his lack of wedding ring was extremely noticeable ... especially since he normally sports a pretty big sparkler.

We got Mariah out earlier this week ... and the 44-year old diva was also walking around without a wedding ring -- only she replaced the expensive hardware with her signature butterfly ring.

Heartbreaker.