Teneo chairman and CEO Declan Kelly, who founded the group in 2011, said: "This transaction ensures we can continue to provide our clients with an ever-growing range of capabilities in this strategically important region."
The fee paid to buy the business has not been disclosed.
Ryan Communication, which was founded in 2010, has offices in Hong Kong, Shanghai and Singapore. The agency specialises in corporate and financial comms, reputation management and investor relations, among other services.
Ryan's senior management team, including founder Damien Ryan, and the agency's 35 staff will all remain in their posts, reporting to Teneo International chairman Charles Watson.
Teneo said it would combine Ryan's three offices with its own teams in Hong Kong and Singapore, creating a "powerful new force in Asia".
Asked whether Ryan would be rebranded, a spokesman told PRWeek: "We will be aligning our brands in Asia later this year."
The acquisition represents the seventh such purchase the agency has made since 2011. Most notably, Teneo acquired PR firm Blue Rubicon in 2015 and combined its entire UK operations into one business under the new name Teneo Blue Rubicon in September last year.
Teneo also acquired UK agencies StockWell Communications Group and Pendomer Communications in 2015, and rebranded both under the Teneo Strategy banner in January 2016.
Teneo Blue Rubicon, which is chaired by former Blue Rubicon senior partner Fraser Hardie, is ranked as the fourth-largest agency in the UK, according to the new PRWeek Top 150 league table.