Sovereign Bancorp, the parent company of Sovereign Bank with its New England headquarters in Boston, will announce today a definitive agreement to acquire Seacoast Financial Services Corp. in an all-stock transaction valued at approximately $1.1 billion.

Sovereign will achieve a Top 4 market share in each of the five fastest-growing counties and the five largest counties in Massachusetts, including the Boston metro market. The announcement will take place at a press conference at 11:15 a.m. at 75 State St. in Boston.

Seacoast, which recently announced the acquisition of Abington Bancorp, is a $5.4 billion asset financial institution and the largest independent bank holding company in Massachusetts. Based in New Bedford, Seacoast has a branch network that spans from the suburbs outside of Boston to the coastal areas of Cape Cod, Nantucket and Martha’s Vineyard. Seacoast is the holding company for CompassBank and Nantucket Bank. CompassBank is a $4 billion state-chartered bank serving Southeastern Massachusetts through 47 full-service branch offices; Nantucket Bank is a $458 million state-chartered bank serving Nantucket Island with three full-service branch offices.

“This is a very strategic transaction for us. If you looked at our geographic footprint in Massachusetts before today, this is the one area where we should build our presence. With Seacoast, we will have a strong position in the high-growth, high-income counties of Southeastern Massachusetts. It is a natural extension of our existing franchise,” said Sovereign Chairman and Chief Executive Officer Jay S. Sidhu. “We are now even better-positioned to take advantage of any disruptions caused by the anticipated Bank of America/FleetBoston merger integration. There are 26 Fleet branches with over $1.8 billion in total deposits within a half-mile of a Seacoast branch. Adding Seacoast will certainly help us achieve our goal of attracting $1 billion of deposits and $1 billion of loans from former Fleet customers.”