Since the 1970s, Keith L. Sachs, the former chief executive of Saxco International, a packaging distribution company, and his wife, Katherine, have been major supporters of the Philadelphia Museum of Art, he as a longtime trustee and she in a number of roles, including adjunct curator.

The couple are also major buyers of contemporary art, working closely with museum curators to amass a top-flight collection of paintings, drawings and sculptures from the 1950s to the present. This week, the museum announced that it had been promised a lion’s share of the Sachs holdings. Included in the gift are 97 works by contemporary masters like Jasper Johns, Ellsworth Kelly, Brice Marden and Gerhard Richter, worth nearly $70 million, according to auction house experts asked for their assessment. Timothy Rub, the museum’s director, said it was one of the most important gifts of contemporary art in the institution’s 138-year history.

“This museum has always been committed to contemporary art,” Mr. Rub said in a telephone interview. “Now, with the Sachs gift, we will have one of the best collections of contemporary art in the country. It’s transformative.”

The gift will fill crucial gaps in the museum’s holdings. There are, for instance, important video works by artists like Allora & Calzadilla, Francis Alÿs, Pierre Huyghe, Steve McQueen, Eve Sussman and Bill Viola, which Mr. Rub described as a “major step forward” for the museum. The collection also helps enrich holdings by artists it already owns, like Mr. Johns and Mr. Kelly. It significantly ramps up its representation of sculptures, too, with work by John Chamberlain, Donald Judd, Cy Twombly, Dan Flavin, Richard Serra and Robert Gober. The artworks include some by the hottest photographers around, including Andreas Gursky, Thomas Ruff and Thomas Struth and Hiroshi Sugimoto.