The agency will serve as Western Union's communication agency of record across Europe and the Commonwealth of Independent States.
APCO won the business following a competitive pitch process. Hill & Knowlton previously held the account.
The agency will co-ordinate the firm's comms activity across a number of European markets, including the UK, France, Spain, Italy and the Benelux countries, and will work with incumbent agencies in other territories.
The brief focuses on corporate affairs, including media relations and thought leadership, as well as public affairs and stakeholder engagement.
Angela Heng, vice-president of corporate affairs for Western Union in Europe, the Middle East, Africa and Asia-Pacific, said: 'Corporate communications is one of Western Union's key priorities.'
APCO is also assisting Western Union with the integration of newly acquired agent network FMT, formerly the money transfer business of Fexco.