Katy Perry just made a massive move on the real estate market ... she's about to close on a $19 million dollar mega mansion in Beverly Hills, TMZ has learned.

The house has 5 bedrooms, 5.5 bathrooms and sits on 5,427 sq feet in a super secure gated community. 

The house was not on the market, and we're told Katy paid the owner a skosh under $19 mil to seal the deal.

The house has all the bells and whistles any star could want -- a long, winding driveway, infinity pool, a gorgeous terrace with a view ... all surrounded by a forest of eucalyptus.

Get this, her neighbors are Adele, Nicole Kidman, Ashton Kutcher and Mila Kunis, Cameron Diaz, Nicole Richie, Zoe Saldana, Ziggy Marley and Guy O'Seary.

Talk about living like a California girl.