A Sinn Féin Assembly member, Michael Ferguson, died suddenly at his home last night.
Party leader Gerry Adams confirmed the death and expressed his sympathies to Mr Ferguson's wife and four children.
Mr Ferguson (53), who was the party's education spokesman and MLA for West Belfast, had been receiving treatment for cancer.
His death came as an interview he had given on his illness was published in a Belfast newspaper.
In a frank full-page interview with the Irish News, he told how he had realised he was unwell last year and had undergone chemotherapy and surgery for testicular cancer. He was preparing for a second round of more gruelling chemotherapy.
Despite his illness, he told the newspaper: "I realise, though, that I am one of the lucky ones. My cancer is treatable."
Mr Adams expressed his deep sympathies to Mr Ferguson's wife, Louise, their four children and his wider family circle.
Ulster Unionist Assembly member Billy Bell also extended his sympathies.
"From our time on Lisburn Borough Council, I knew Michael to be not only an energetic political opponent, but more importantly he was a dedicated family man," Mr Bell said.
He also praised Mr Ferguson's decision to go public in an effort to raise awareness of the illness.
Frank Bunting, the northern secretary of the Irish National Teachers' Organisation, described Mr Ferguson as a friend of the profession who had a passionate commitment to helping children from socially disadvantaged backgrounds.
A Queen's University graduate, he was elected to Lisburn Council in 1999.
Sinn Féin Assembly group leader John O'Dowd said Mr Ferguson brought a huge energy and determination to his work as an elected representative.
Patricia Lewsley of the SDLP said Mr Ferguson was a colourful character who would be missed by many members of the political family.
"He was honest and straightforward and I can truly say that he was somebody I could always work well with on bread-and-butter issues despite our considerable political differences.
"Many of us learned of his death as we were commenting on how very brave he was to go public on the nature of his illness in order to raise awareness of testicular cancer. The best memorial we can give him is for all of us to carry on the job that he started," she said.