Smith is currently head of media at the Federation of Small Businesses, which claims to be the UK's largest campaigning pressure group, with around 200,000 members and 188 branches across the UK.
She has held the FSB role since 2012, before which she spent almost 20 years as a journalist and editor with the BBC.
Smith will move to WUBS in the new global role on 23 November. She will be based in London. A press release from the firm says she will be "responsible for implementing and building a strategy across multi-channels, to drive brand awareness, and will help to create a full understanding of WUBS, across the organisation and news media, globally".
She will report to Dan Diaz, vice-president of global corporate comms, who said: "At Western Union, we are actively listening to the voice of consumers and businesses worldwide and using that feedback as the basis for innovation in all cross-border money transfer and payment capabilities.
"Liesl is an experienced communications executive, with a track record. She is known for her strong business communication sense and we look forward to Liesl’s leadership in telling our global b2b story."
Smith said: "This is an exciting time to join the WUBS comms team. The number of new businesses continues to grow and advancing technology presents new communication challenges and opportunities. It is crucial businesses and organisations all over the world get access to the money they need, so they can grow their companies, increase exports and create the jobs of the future."
The FSB has begun the search for a successor.