Jacqueline Kennedy Onassis has bought 375 acres of moors, marshes, ponds and beachland in this holiday resort for $1.15 million to save them from developers, her attorney said yesterday.

In a telephone interview, Alexander D. Forger, one of Onassis' New York lawyers, said she intends to preserve "the unique beauty and wild state" of the land in Gay Head at the southeastern tip of the Vineyard, a fashionable holiday island about eight miles off the Massachusetts coast.

He said that Onassis actually bought the property last January but declined to make any announcement "in the hope that the tranquility of the island might be spared the intrusions and publicity that so frequently accompany her life."

Forger said that by acknowledging the sale, rumors and unwanted attention about it will now end.

Onassis plans to build only one house on the land running along Squibnocket Pond and down to the oceanat Long Beach. It also includes two smaller ponds, Black Pond and Lily Pond.

The lawyer said, "Mrs. Onassis hopes that this track will remain with her family's ownership and that it will retain its natural beauty and continue as a wildlife sanctuary."

Forger did not say when a structure would be built on the land, which also stretches for almost a mile along the southeastern shore of Gay Head, a sparsly-settled section of Martha's Vineyard.

Forger said that when Onassis first saw the land's "moors, dunes, ponds and marshes, she was determined to preserve its unique beauty and wild state.

She also wanted to save the land from inevitable developing by a purchaser who might have built about a dozen houses on the land.

The land was bought from the Hornblower family and does not conflict with lands in Gay Head that are claimed by Indian groups.