Massachusetts-based Cambridge Trust Co., which earlier this year bought Portsmouth’s Optima Bank and Trust, has announced it is buying another bank in an all-stock transaction worth $122 million.

The deal, announced Dec. 5, will see Cambridge Trust absorb Wellesley Bank, headquartered in Boston’s western suburbs and serving a similarly well-heeled clientele. The purchase price comes to $45.54 per Wellesley common share based upon Cambridge Bancorp’s 10-day average closing price of $78.53 as of Dec. 4.

Cambridge Trust had total assets as of Sept. 30 of $2.8 billion. Wellesley Bank has assets of about $980 million.

The same day the merger was announced, Cambridge Bancorp announced it was offering 479,000 shares of common stock to the public at a price of $73 per share. If successful, the offering will raise $33.2 million for the company after underwriting discounts and commissions. In a statement, the company said it plans to use the proceeds for “general corporate purposes, including funding organic growth and potential acquisitions.”

“We are pleased to announce the strategic combination of Cambridge Trust and Wellesley Bank, who are both dedicated to providing individuals, families and businesses with exceptional personal attention and custom financial solutions,” Denis Sheahan, CEO of Cambridge Trust, said in a statement. “Wellesley Bank is a well-managed, financially strong and growing company located in attractive markets. The combination strengthens the position of Cambridge Trust in Greater Boston and is a logical extension of our market. We look forward to welcoming the talented Wellesley Bank team to Cambridge Trust.”

Thomas Fontaine, chairman, president and CEO of Wellesley Bank, will join the combined company in the role of chief banking officer and director. He will also be one of three Wellesley directors to join Cambridge Bancorp’s board of directors.