Ciara: So, That Was a Bachelorette Party!!!

Ciara's bachelorette went down 2 weeks ago in Vegas ... and no one connected the dots.

Ciara's BFFs, Serena Williams and Kelly Rowland, flew in for the weekend rager at the Wynn hotel. The crew -- about a dozen deep -- all agreed to keep it mum. Some signed NDAs.

As for what they did ... they hung at the pool, went to Intrigue nightclub, and also took in the aquatic show Le Reve.

Brazilian soccer star Neymar was partying at a nearby bungalow and stopped by to say hi.

TMZ broke the story ... Ciara and Russell are set to take the big plunge Wednesday in Liverpool.