NEW YORK (AP) _ The Metropolitan Museum of Art unveiled on Thursday a new $26 million wing for its collection of 20th century art and named it for Lila Acheson Wallace, the cofounder of Reader's Digest and one of the museum's greatest benefactors.

The two-story southwest wing has 110,000 square feet of floor space. Works of art will be installed by next year, said William B. Macomber, museum president.

Mrs. Wallace, who died in May 1984, established charitable foundations that provided $11 million for the wing, according to Philippe de Montebello, director of the museum. New York City contributed $8 million and the museum funded $7 million, he said.

The annual operating costs of the wing are estimated to be $2.5 million, of which Mrs. Wallace's contributions will cover about half in perpetuity, according to John Ross, a museum spokesman.

The museum's 20th century collection comprises 8,219 paintings, works on paper, sculpture and a design and architecture collection, Macomber said. It includes such masterpieces as Picasso's portrait of Gertrude Stein (1906); Matisse's ''Nasturtiums and 'The Dance''' (1912); Grant Wood's ''The Ride of Paul Revere'' (1931) and Salvador Dali's ''Crucifixion'' (1954).

The wing will open to the public in February 1987.