The official’s office has been searched as part of an inquiry into bribery, fraud and money laundering.
Police have raided the London headquarters of Unite, one of the largest trade unions in the UK.
A Unite spokesperson confirmed one of its employees is subject to a criminal investigation by the police.
They added that officers searched the employee’s office, at the union’s Holborn headquarters, under warrant on Wednesday.
It is understood that a number of officers from South Wales Police were involved in the operation.
The force said it had searched a number of addresses as part of a joint investigation with HM Revenue and Customs into allegations of bribery, fraud and money laundering.
It added the addresses were in the South Wales, Metropolitan, Merseyside, Cheshire, North Wales, Dyfed Powys and Northamptonshire police force areas.
South Wales Police said that “a number of suspects will be interviewed in due course”.