The Gate won the business following a pitch against undisclosed agencies, and will be responsible for creating a series of advertising campaigns incorporating outdoor, press and digital channels.
The campaigns will run in the major markets that Threadneedle operates in, which includes the UK, Germany, Italy, Hong Kong and Taiwan.
The account will be led by The Gate's London office, with support from its partner agency WOB, an integrated advertising shop based in Germany.
David Rowe, the head of wholesale and institutional marketing at Threadneedle, made the appointment.
Threadneedle has worked with a number of ad agencies in recent years, including Doner Cardwell Hawkins and Citigate.
Rowe said: "Given Threadneedle's global reach, it is crucial to have an agency that has a deep understanding of financial products, strong creative direction and can also operate comfortably across numerous jurisdictions."