Fortis Insurance has been appointed by Aon to underwrite Post Office travel insurance products and to administer claims services.
The two-year contract, for which a total of five insurers competed, commences on 2 June.
Aon Personal managing director Tony Martin said: "We were very impressed with the enthusiasm and professionalism of the Fortis team. Their track record in creating and maintaining customer satisfaction levels was a key factor in the selection process.
"With its experience of insuring 50 million travellers over the past 35 years, Fortis is a good strategic fit for our partnership with the Post Office."
Post Office travel and leisure category manager, Kevin McAdam, said: "We are excited about the prospect of working with Fortis. We know it by reputation and look forward to working with it in expanding our travel insurance business further."