It was reported 15 to 20 officers attended the offices on Wednesday, leaving with boxes of files and a computer.
A Unite spokesperson told Sky News: “A Unite employee is subject to a criminal investigation by the police.
“On Wednesday 6th April, the employee’s office at Unite HQ, in Holborn London, was accessed and searched by the police under warrant.”
The raid was conducted by officers from South Wales Police, according to the broadcaster.
There were two police vans and unmarked police cars at the scene.
Unite is one of the UK’s largest trade unions.
A spokesperson for South Wales Police said: “South Wales police has executed warrants as part of an investigation into allegations of bribery, fraud and money laundering.
“The warrants were executed on Wednesday 6th April at addresses in South Wales, Metropolitan, Merseyside, Cheshire, North Wales, Dyfed Powys and Northamptonshire police force areas.
“The joint investigation with Her Majesty’s Revenue and Customs is continuing and a number of suspects will be interviewed in due course.”