Dave Prentis, general secretary of Unison, has become president of the Trades Union Congress.
Mr Prentis called on the government to "act against rogue bosses"
He was elected as the TUC's annual gathering in Brighton drew to a close.
Mr Prentis was already a member of the TUC's general council and executive, and is a director at the Institute of Public Policy Research think-tank.
He promised to lobby the government to help "vulnerable workers" in the UK who were enduring "poverty wages, excessive hours and shocking ill-treatment".
He succeeds Alison Shepherd, who is a member of Unison and has been president of the TUC for the past 12 months.