Ariana Grande and Cynthia Erivo proved their "Wicked" run isn’t over for good ... reuniting for a surprise performance in London.

Cynthia joined Ariana during Sunday's "Eternal Sunshine” show at the O2 Arena, where the costars performed "For Good" before launching into "Get Happy / Happy Days Are Here Again."

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The two embraced as the crowd went wild ... marking their first performance together since last November's "Wicked: One Wonderful Night" TV special, where they sang the same two numbers.

Cynthia had already revealed she planned to attend one of Ariana's London concerts ... but apparently she wasn’t content watching from the crowd.

The reunion came during the second show of Ariana's 10-night run at the O2, which will bring her tour to a close September 1.

Oz may be in the rearview ... but Ariana and Cynthia seemingly haven't sung their last note together.