The veteran campaigner, whose near two decades in jail failed to dull his commitment to the democratic cause, had suffered worsening ill health in recent weeks.
He died in hospital in Yangon early Monday, National League for Democracy party spokesman Nyan Win told AFP. A funeral service will be held on Wednesday.
A towering figure within the democracy movement, Win Tin formed the NLD with Aung San Suu Kyi in 1988 and was imprisoned the following year in the wake of a student-led pro-democracy uprising.
"We are so sad to have lost him -- it is like the world has been lost," he told AFP.
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"But we have many things to do. We will continue as he asked and will follow his way to democracy," he added.
Myanmar began its emergence from nearly half a century of military rule in 2011, under a quasi-civilian government that has won international plaudits for reforms including the release of hundreds of political prisoners.
Suu Kyi, who was freed from years of house arrest in 2010, has also been welcomed into parliament at the helm of her party and has indicated her wish to become president after 2015 elections.
Win Tin was freed by the former military junta from Yangon's notorious Insein prison as part of an amnesty in September 2008.
During his imprisonment he was interrogated for up to five days at a time, deprived of sleep, hooded and beaten, and said torture at the hands of the authorities was routine.
From 1996 he was also kept in solitary confinement, allowed only fleeting 15 minute visits from family every two weeks.
Last year he told AFP that he continued to wear a blue shirt in solidarity with dissidents still in jail and to show the world that his country was still not truly free.