Two small, depositor-owned banks on the South Shore have agreed to merge.

The mutual holding companies of Weymouth-based Equitable Bank and East Weymouth-based Coastal Heritage Bank announced the deal Tuesday. The new entity will continue under the holding company Equitable Bancorp, but the combined bank will continue as Coastal Heritage Bank.

With Coastal Heritage’s roughly $517 million in assets and nine branches, and Equitable’s roughly $332 million in assets and six branches, the new entity will have nearly $850 million in assets and 15 branches, assuming there is no consolidation.

“Equitable Bank and Coastal Heritage Bank proudly share a rich history of being two of the oldest surviving co-operative banks in Massachusetts,” Donald P. Gill, Coastal’s president and CEO, and Robert W. Terravecchia Jr., Equitable’s president and CEO, said in a joint statement. “Both banks share a common culture and strong commitment to the communities we serve.”

The deal essentially represents a merger of mergers and the culmination of what has been a busy few years for both institutions.

Coastal Heritage Bank and its holding company merged with Scituate Federal Savings Bank and its holding company in December 2015. Equitable Bank, which was then based in Lynn, merged with Weymouth Bank in June 2016, taking Weymouth Bank’s headquarters and charter but continuing under Equitable Bank’s name.

Both banks do not appear to have grown much since then – the new Coastal entity at the time of its merger with Scituate Federal was expected to have about $500 million in assets, while the new Equitable entity at the time of the merger with Weymouth Bank was supposed to have about $330 million in assets.

Under the new leadership structure, Joseph C. Hayes, Coastal’s chairman of the board of directors, will serve as chairman of the board of Equitable Bancorp and Coastal Heritage Bank. Gill will serve as president and CEO of each entity.

Terravecchia will serve as vice chairman of the board and executive vice president of each entity and will become president and CEO of each entity upon Gill’s retirement, which is expected in April of 2020.

The transaction is expected to close in the first quarter of 2019.