Wedding crashers aren't usually well-received when they're discovered ... unless of course it's Zendaya barging in!

The actress showed up to a marriage ceremony in Las Vegas Saturday as part of the promotion for her new movie "The Drama" ... cheering loudly for the happy couple and even signing their marriage document as an official witness.

The star stopped to pose for pics with the bride and Alana Haim, who seems to be acting as a wedding DJ at the venue ... with Zendaya flashing a smile while standing in front of a fabric heart with the ladies.

Zendaya made the appearance at a wedding chapel that A24 -- the production company behind "The Drama -- opened for couples to get married in quickie ceremonies.

The chapel opened at 10 AM PT Saturday ... and will be conducting ceremonies -- by reservation only -- until midnight.

Zendaya went to this wedding solo ... leaving her fiancé -- or possibly husband? -- Tom Holland behind.

"The Drama" stars Zendaya and Robert Pattinson as a couple about to get married ... before secrets threaten their union the week of the ceremony. It's set to come out April 3.

A wedding day is always unforgettable ... especially when Zendaya's cheering you on.