The city on Tuesday designated the Ambassador Grill as a landmark. View Full Caption KRJDA/Ronald Livieri

MIDTOWN EAST — The disco-era interior of a Midtown East hotel restaurant was made a city landmark Tuesday.

The Landmarks Preservation Commission protected the Ambassador Grill inside the ONE UN New York Hotel at East 44th Street and United Nations Plaza.

Its interior has been called a "time capsule" from 1975 and represents a key moment in the city’s architectural history, advocates say. Preservationists have lobbied for it to be landmarked for months.

Featuring flashy, angled mirrors on its walls and ceilings, it was the work of architect Kevin Roche and has been hailed as a relic of a moment in time at which architects were turning away from the staid, minimalist modernism of the 1950s and 1960s and toward the more eccentric postmodern era, according to Theodore Grunewald, a preservationist and trained architect.

“It’s about time we have landmark status for building interiors that are representative of the past 30 or so years,” Grunewald said in September, when the LPC agreed to consider the restaurant.

“It’s also just a spectacular space.”

The owner of the hotel closed the restaurant for renovations in November 2015, but preservationists began agitating for its protection after Grunewald said he snuck into the Ambassador Grill and saw what looked like shoddy construction that threatened its historic character.

Preservationists tend to have a tougher time winning landmark designation for interior spaces which often see more alterations over time than building exteriors, but the Ambassador Grill is noteworthy for retaining nearly all of its initial design elements.

A representative of the hotel did not immediately respond to a request for comment.