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.