Seaport Square master developers Boston Global Investors and Morgan Stanley have sold 12.5 acres on the South Boston waterfront to Chestnut Hill-based W/S Development.

The parcels, which stretch from Northern Avenue to Summer Street, are the last available developable tracts within the master-planned Seaport Square that is reshaping the waterfront. The properties were sold for $359 million.

They’re preapproved for 2.8 million square feet of development, including 1.25 million square feet of residential buildings, 500,000 square feet of stores and restaurants, 400,000 square feet of office space and three hotels. The land also includes a park currently being built between Seaport Boulevard and Northern Avenue, and the completed Q Park on Stillings Street.

W/S already has a major stake in the 23-acre mixed-use development as owner and leasing agent for 250,000 square feet of retail space on the lower levels of One Seaport Square, a $600 million apartment complex under construction on Seaport Boulevard. It also will own 125,000 square feet of retail space in three 22-story residential towers on parcels M1 and M2 at the corner of Seaport Boulevard and East Service Road, which are under agreement to Chinese real estate investor Celona Capital. That project is expected to start in early 2016. W/S also owns retail space in new Skanska USA’s 101 Seaport office tower, which is occupied by PwC, and 121 Seaport, currently under construction on speculation.

“We understand there was some competition, but because of our longstanding relationship with [BGI and Morgan Stanley], we definitely had the inside track,” said Richard Marks, a principal with W/S.

The acquisition, scheduled to close today, is the biggest in WS’s history. The company specializes in development and leasing of shopping centers throughout New England, including such recent projects as the 406,000-square-foot Street At Chestnut Hill.

An undisclosed North American pension fund is an equity partner on the transaction, Marks said. The parcels include D, F and G on Northern Avenue, L3 through L6 north of Congress Street and N and P south of Congress Street.

It’s not clear in which order the parcels will be developed, but the buildout will be market-driven, Marks said. W/S has not decided whether to partner with office and residential developers on those portions of the project, he said.

Developer John Hynes’ BGI and Morgan Stanley bought the 23-acre property from News Corp. in 2006 for $204 million.