The Ludwig Mies van der Rohe-designed Seagram Building at 375 Park Ave. is widely regarded as the masterpiece of the International Style.

But there’s one thing the 630,000 square-foot tower, opened in 1958, never had: an outdoor terrace.

Owner Aby Rosen’s RFR Realty has just opened one on an 11th-floor setback roof deck on Seagram’s east side.

The executive lounge and terrace designed by Studios Architecture is exclusively for tenants’ use. It’s part of a $25 million program to pamper tenants spending up to $170 a square foot. “We’re not the cheapest option in town,” Rosen chuckled.

The “club-like” facility includes a glass-enclosed library offering coffee and tea service, wine tastings, and food from the Four Seasons and Brasserie restaurants downstairs.

The 7,280 square-foot alfresco deck boasts 4,500 square feet of grass. At night, ambient light will pour from skyscrapers that loom close enough to give it the feel of an open-air room.

The terrace “will let executives spend time away from their offices to meet friends and clients in an environment that’s disarming,” Rosen said.