Len McCluskey has won the poll to be re-elected as general secretary of Unite, beating his nearest rival by over 5,500 votes.
Mr McCluskey polled 59,067 votes, with Gerard Coyne securing 53,544 and Ian Allinson 17,143.
The turnout was just over 12%, the Unite union said.
Unite's acting general secretary Gail Cartmail said: "I congratulate Len McCluskey on his victory and would urge the entire union to pull together in the interests of our members, and not least to work for a Labour victory in the general election.
"The turnout in this important election can give no cause for satisfaction and, while the tone of the campaign will not have helped, the underlying reason remains the archaic and expensive balloting system imposed on trade unions by law.
"The sooner we can more to secure and secret workplace and online voting the better for union democracy."