Beyonce's sister Solange went AFTER her brother-in-law Jay Z last week in an elevator, and the entire thing was caught on video.

 TMZ obtained the video and reports the incident happened at a Met Gala after-party last week in New York City.

You can see in the video Jay Z, Beyonce and Solange step into the elevator with a bodyguard, and almost immediately after walking in Solange gets in Jay Z's face and goes after him.

The bodyguard tries to hold Solange back, but she connected a few times on Jay Z. Beyonce appears to later talk to her sister, but stays calm throughout the entire thing.

It's still unclear on what ignited the fight.

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