The move is the latest push by a large US payment processor to expand into prepaid cards, in an attempt to boost revenues from processing smaller dollar transactions.
In June, rival American Express Co announced plans to sell a prepaid card online.
Prepaid cards are exempt from new US rules that limit the processing fees that card companies can charge merchants.
The cards allow consumers to load a card with cash at any one of Western Union's 485,000 agent locations around the world. And Western Union customers can also load money onto a prepaid card, instead of sending cash.
The money on the card can be spent anywhere that accepts MasterCards.
MasterCard — through Western Union — has offered US prepaid cards for the last year, and has issued 1.2 million cards so far, executives said.
MasterCard Chief Executive Ajay Banga and Western Union CEO Hekmet Ersek declined to discuss how many cards they expected to issue worldwide within the next year.
Banga said, however, the total would be "orders of magnitude" higher than the US total so far.
