Financial terms of the agreement were not immediately available.
Ryan Labs has approximately US$5.1 billion in assets under management for clients in the United States.
The deal is expected to close in the first quarter.
Ryan Labs will retain its brand name and management team as it operates as a unit of Sun Life Investment Management.
The firm specializes in investment strategies for defined benefit pension funds and other institutional clients.
"Bringing an established and innovative firm like Ryan Labs into our family is a significant first step in building Sun Life Investment Management's third-party business in the U.S.," said Steve Peacher, president of Sun Life Investment Management and chief investment officer for Sun Life Financial.
"They have a first-rate track record and share our focus on bringing value for liability driven investors."