A turn-of-the-century Upper East Side mansion is hitting the market for $34 million — down from its 2015 asking price of $39 million.

Known as the Waterfall Mansion, the five-story former carriage house on 80th St. boasts a 23-foot, indoor waterfall. It’s currently used as an art gallery with living quarters for artists in residence on the upper floors.

It can also be transformed back into a single-family mansion.

The seller is Kate Shin, who worked in private equity where she was nicknamed “The Bomb” for deals including the Chelsea Market project.

Over the years, she says the home has served as an event space for Christie’s Auctions, Gucci, Rolls Royce, Fendi and the United Nations.

The five-bedroom, 6½-bathroom home has also been on the market as a $100,000-a-month rental.

It features a professional-grade kitchen.

The listing brokers are Howard Morrel and Leslie Hirsch of Compass, who also listed and sold the Halston House, on 63rd St., to designer Tom Ford.