The parent of Rockland Trust Co. is acquiring Somerville-based Central Bank in a deal valued at $54.8 million, expanding Rockland Trust’s presence in Middlesex County.

Rockland Trust, the commercial bank subsidiary of Hanover-based Independent Bank Corp., has about $5 billion in assets, 67 retail branches, 10 commercial lending centers and four mortgage banking centers in Eastern Massachusetts and Cape Cod, according to its website.

Central Bank, a subsidiary of Central Bancorp Inc., is a 97-year-old cooperative bank with nine branches in Somerville, Arlington, Burlington, Chestnut Hill, Malden, Medford, Melrose and Woburn.

"Our customers will benefit from the strength that Rockland Trust offers and can look forward to a greater range of products and services, " John D. Doherty, Central Bank’s chairman and CEO said in a statement.

Rockland Trust President and CEO Christopher Oddleifson said the acquisition will "significantly increase Rockland Trust’s presence in the communities of Middlesex County."

The transaction has been approved by the boards of directors of each company and is subject to regulatory approval. The merger is expected to close in the fourth quarter.

Independent Bank Corp. was advised by Sandler O’Neill & Partners LP, with Mary Anne Callahan as lead investment banker, and used Choate Hall & Stewart LLP as its legal counsel for the transaction. Central Bancorp was advised by Keefe, Bruyette & Woods Inc., with Benjamin H. Saunders as lead investment banker. Its legal counsel was Kilpatrick Townsend & Stockton LLP.