Marking the largest conservation purchase on the Vineyard in recent memory, the Sheriff’s Meadow Foundation and Martha’s Vineyard Land Bank announced Thursday that they will buy 304 acres of undeveloped land at Red Gate Farm, the historic, ecologically rare oceanfront estate in Aquinnah owned by the family of Caroline B. Kennedy.

The purchase price is $27 million for two distinct sections of the property. The Kennedy family will retain ownership of 95 acres in the center of the property, where their homes are situated.

A purchase-and-sale agreement was signed this week, with a closing set for December, leaders at both Sheriff’s Meadow and the land bank said.

The land bank will contribute $15 million to the purchase, while Sheriff’s Meadow will contribute $12 million.

The purchase will include a mile of pond frontage and half a mile of Atlantic-facing beach. The property, which will be renamed Squibnocket Pond Reservation, will be open to the public after a yearlong biological study, both organizations said.

Red Gate Farm encompasses some 340 acres of stunningly beautiful windswept coastal dunes, wetlands, hillocks and salt-blasted heathlands between Moshup Trail and Squibnocket Pond. The Massachusetts Natural Heritage and Endangered Species Program has described the property as one of the most important tracts of land in the commonwealth.

It served as a private retreat for Jacqueline Kennedy Onassis until her death in 1994, and has remained family-held until now.

About a year ago the Kennedy family put much of the property on the market, with an asking price of $65 million.

The purchase is being called below-market because it is less than the appraised value of the land, Sheriff’s Meadow executive director Adam Moore said.

It also marks a first for the land bank and Sheriff’s Meadow, who will jointly hold title to the property, based on their proportional shares of the purchase price (56 per cent and 44 per cent respectively).

“This is perhaps the most significant acquisition in the history of Sheriff’s Meadow Foundation, Mr. Moore said. “It’s very similar to the 1967 effort led by Henry Hough to preserve Cedar Tree Neck Sanctuary. It’s a wonderful thing for all the people of Martha’s Vineyard and all the generations to come who will all be able to enjoy an appreciate the beauty of this land.”

He said Sheriff’s Meadow has already raised $9 million of the $12 million it will contribute toward the purchase. Another $1 million matching donation has been put forward by a donor, he said.

Land bank executive director James Lengyel also underscored the significance of the purchase.

“Someone said this is why we created the land bank,” and I think that’s true, Mr. Lengyel said. “With a property of this nature you grope for the right adjective to describe it, and the best word I can come up with is singular — it’s a singular property.”

He said the land bank will borrow the $15 million from the Martha’s Vineyard Bank, making the purchase even more affordable than if the land bank issued a revenue bond, which it is legally authorized to do — and also making it an all-local transaction. “I’ve got a lot of thank-you’s to make and a very big thank you goes to the MV Bank and [bank president] James Anthony,” Mr. Lengyel said.

Most of all, he credited the Kennedy-Schlossberg family.

“You’ve got to give credit where credit is truly due,” Mr. Lengyel said. “They had many, many, many options and they chose conservation.”

Mr. Moore said conversations with the family have been under way for most of the last year, and negotiations began in earnest this spring, with the land bank included.

Land bank revenues come from a two per cent transfer fee on most arm’s length real estate transactions, with the money used to buy conservation land. Sheriff’s Meadow is a private, nonprofit land trust.

Mrs. Onassis bought Red Gate Farm in 1978 for $1.1 million from the Hornblower family, saving it from potential development and creating a refuge for herself from an exceedingly public life.

She and her daughter fiercely protected the land over the decades.

The property originally incorporated 31 lots, but in 2005 the family consolidated it into seven lots in a limited subdivision plan that anticipated separate parcels for each of Ms. Kennedy’s three children. The plan was approved by the Martha’s Vineyard Commission a year later.

In 2013, Ms. Kennedy and her husband Edwin Schlossberg gifted an additional 30 acres north of Moshup Trail to the Vineyard Conservation Society. The society has been engaged for many years in an initiative to protect and conserve the globally-rare heathlands along remote Moshup Trail, a ribbon of road that curves through the extreme southwestern edge of Aquinnah with its wildlands and sweeping views of the Atlantic Ocean.

Two other lots were placed on the market the same year — one fronting the ocean and another fronting Squibnocket Pond — but were never sold.

Mr. Lengyel said the town of Aquinnah also played an active role in the purchase, among other things through the town land bank advisory board, which will take the lead in managing the property. A draft management plan calls for creating trails, beach access, a kayak launch, limited parking and two trailheads, one coming off State Road and another off Moshup Trail.

Sheriff’s Meadow president Peter Getsinger echoed remarks about the significance of the purchase, and he singled out the new partnership between Sheriff’s Meadow and the land bank as an important forward-thinking step.

“This is the first time we have done anything of this size with the land bank, and I think given what’s happening with land valuations on the Vineyard, this partnership in conservation could be a blueprint for the future,” Mr. Getsinger said. “We are two organizations with different funding sources but our missions are very similar. Hopefully we can build on this.” He continued:

“We’ve got a race on right now, it’s a race between conservation and trying to keep the Island as it wants to be, against the development that is on the rise.”

He also said despite the fact that Sheriff’s Meadow has raised most of the money for the purchase, more funds will be needed, both to complete the purchase and create an endowment for maintenance of the property.

“We want to see a lot of small donations going forward,” Mr. Getsinger said. “We have more to raise and I have no doubt that we will raise it, but we want people to know we have more work to do. I don’t want people to think they don’t need to give.”

Finally, he said he hoped the purchase would inspire others, including owners of family-held lands, to make conservation a top priority.

“I hope that what we are seeing is a new movement in conservation on Martha’s Vineyard,” Mr. Getsinger said. “This Island needs to be protected, not exploited. I’m thrilled with the announcement today.”

More pictures.