Sun Life Financial Inc. is a Canadian financial services company headquartered in Toronto, Ontario. Founded in 1865, it operates internationally in life insurance, wealth management, and asset management. As of 2024, the company manages over CAD$1.3 trillion in assets and serves clients in Canada, the United States, Asia, and other markets.

Sun Life is one of the largest life insurers in Canada and ranks among the top global insurers by market capitalization. It is publicly traded on the Toronto (TSX: SLF) and New York (NYSE: SLF) stock exchanges.

Founded in Montreal, Quebec, as The Sun Insurance Company of Montreal in 1865 by Matthew Hamilton Gault (1822–1887), an Irish immigrant who settled in Montreal in 1842. However, operations actually began in 1871. By the end of the 19th century, it had expanded to Central and South America, the United States, the United Kingdom, West Indies, Japan, China, Philippines, India, North Africa and other international markets. During the next five decades, the company grew and prospered, surviving the difficulties of World War I and the large drain on its finances through policy claims arising from the large number of deaths caused by the Great Flu Epidemic of 1918.

The company's original Dominion Square building in Montreal was built in 1918. Capping a Montreal construction boom that began in the 1920s, the company completed construction of the expansion of its headquarters with its new 26-story headquarters north tower in 1933. Although the head office of the Royal Bank of Canada on St. James Street was taller by several floors, the Sun Life Building was at the time the largest building in terms of square footage anywhere in the British Empire.

Chartered in 1865, its traditional base business worldwide remains life insurance. However, it now also has significant asset management operations. In 1919, it was the first Canadian company to offer group life insurance. Having begun its first US operations in 1895, the company sold its first group life plan in the U.S. in 1924.^{[citation needed]}

In World War II, as part of Operation Fish, securities from the United Kingdom were secretly moved to the Sun Life Building for safekeeping. A persistent, but incorrect, rumour is that the Crown Jewels of the United Kingdom had been illegally shipped out of the United Kingdom during WWII and stored there was deliberately spread in Montreal to account for increased activity at the building.

In 1977, the newly-elected sovereigntist Quebec government passed the Charter of the French Language (known as Bill 101), making the use of French language mandatory for medium- and large-scale companies when communicating with French-speaking staff. The law was received negatively by the English-speaking business community, many of whom perceived that the historical rights of the English-speaking minority should be respected. The new government also promised a referendum on Quebec sovereignty, injecting instability into the Montreal business community. On January 6, 1978, Sun Life became the first large company to leave Quebec post-election, announcing that it would move its head office to rented space in Toronto, Ontario. Officially, Sun Life said it was motivated by the political instability and economic uncertainty of Quebec's future, but skeptics said it was the company's unwillingness to comply with the requirements of Bill 101. In a reaction the next day, the province's finance minister Jacques Parizeau called Sun Life "one of the worst exporters of Quebec capital", and threatened to freeze some $400 million CAD in assets.

In 1979, the company acquired a property at University Avenue and King Street in downtown Toronto and constructed a new office complex, the Sun Life Centre, which was completed in 1984.