TOKYO—Sumitomo Life Insurance Co. said Tuesday it has agreed to buy U.S. life insurance company Symetra Financial Corp. for $3.73 billion, the latest large acquisition in the U.S. by a Japanese life insurer.

Sumitomo Life, an unlisted mutual company, is Japan's fourth-largest insurer by premiums. Like its rivals in Japan, it has been looking for acquisition opportunities overseas because of a shrinking and aging population at home.

Symetra Financial, based in Bellevue, Wash., offers life insurance and retirement plans. The company had total assets of $34 billion as of June 30, and it posted net income of $254 million in 2014.

Sumitomo Life said it plans to keep Symetra's management team after the transaction closes, which is expected to occur in the first half of 2016. Symetra's board unanimously approved the deal, Sumitomo Life said.

With the purchase, Sumitomo Life said it "will be able to enjoy growth opportunities in the U.S. life-insurance market, in which further growth in population and high growth potential of premium income per capita are expected."

Sumitomo Life's bigger rival, Meiji Yasuda Life Insurance Co., last month announced a takeover worth about $5 billion of Oregon-based StanCorp Financial Group.

Dai-ichi Life Insurance Co. closed a deal earlier this year to acquire U.S. insurer Protective Life Corp. for $5.6 billion. In June, Tokio Marine Holdings Inc., Japan's largest nonlife insurer by market value, agreed to buy U.S.-based HCC Insurance Holdings Inc. for $7.5 billion.

Symetra's largest shareholders are White Mountains Insurance Group Ltd., with 18%, and Warren Buffett's Berkshire Hathaway Inc. with 17%. In a statement, Mr. Buffett said Symetra Chief Executive Thomas Marra "executed a great deal for shareholders."

Write to Peter Landers at peter.landers@wsj.com

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