The Communication Workers Union (CWU) has confirmed it is backing Ms Dugdale for the position as well as fellow MSP Alex Rowley for the post of deputy.
Ms Dugdale faces competition from Ken Macintosh for the top job left vacant by Jim Murphy, who stood down in the wake of the party’s devastating defeat in Scotland in the general election.
Mr Rowley is vying for the deputy leadership alongside Richard Baker MSP and Glasgow City Council leader Gordon Matheson.
A spokesman for the CWU, which represents workers in post, telecoms, mobile and financial services companies, said: “It is clear that the Labour Party needs to rebuild in Scotland and the CWU believes the best path back to recovery will be under a new leadership team consisting of Kezia and Alex; and we are recommending our members to support them.”
The endorsement follows that of Unison Labour Link, the affiliated section of Scotland’s largest trade union, last week.