Ariana Grande was confronted by an unruly fan who leaped a barricade and grabbed her at the “Wicked: For Good” premiere in Singapore ... and her costar Cynthia Erivo quickly jumped in to protect her!

It all went down Thursday ... as Ariana, Cynthia, Michelle Yeoh and Jeff Goldblum were all hand-in-hand, greeting fans on the yellow carpet at Universal Studios Singapore.

Check out the video ... as Ariana greets a fan, a man on the opposite side suddenly jumps the barricade and beelines straight for her -- throwing his arm around her and jumping up and down with a big smile. X/@lelandweszka

Needless to say ... Ariana and Michelle both look shocked, but Cynthia wastes no time getting between the wild fan and Ariana -- putting hands on his shoulders and forcing him off AG.

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