A new Mondrian hotel is slated to open in September, The Post has learned.

Mondrian Park Avenue, from developer David Moinian, will open in a converted and expanded office building at 444 Park Ave. South, sources said.

The restaurant and bar on the lobby/street level will seat 125 people. The downstairs nightclub seats around 100 people. There’s also a rooftop lounge that seats 100 people inside and 100 people outside.

The new Mondrian will be operated and managed by Journal Hotels, a new label created by veteran hotelier and Journal Hotels CEO Stephen Brandman, along with the owners of the Hollywood Roosevelt hotel.

Journal’s management portfolio includes Ian Schrager’s former Public Hotel in Chicago — now called the Ambassador Hotel — as well as the Hollywood Roosevelt, Two Bunch Palms in Palm Springs, Hotel G in San Francisco and the MacArthur in Los Angeles.

The Mondrian brand is part of sbe, a hospitality company that develops, manages and operates hotels and residences including SLS Hotel & Residences, Delano, Redbury, Hyde Hotel & Residences, Clift, Hudson, Sanderson and St Martins Lane; as well as restaurants and lounges including Katsuya, Cleo, The Bazaar by José Andrés, Fi’lia by Michael Schwartz, Umami Burger, Hyde Lounge and Skybar.

Brandman could not be reached for comment.