Simon Cowell's GF has been walking around with a giant rock on her finger -- and last night, he finally confirmed that yes, it is an engagement ring.

Mezghan & Simon: Click to watch
Simon and his now-fiancee Mezhgan Hussainy were on Leno last night where the harsh "American Idol" host joked he proposed because he "made a decision this year to make somebody happy."

Mezhgan is Cowell's make-up artist.

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