Peter, Paul and Mary singer Peter Yarrow is dead after a long battle with cancer ... TMZ has confirmed.

Peter died Tuesday at his home on Manhattan's Upper West Side ... according to his publicist, Ken Sunshine, who tells us Peter's cause of death was bladder cancer.

We're told Peter was diagnosed with cancer four years ago, and battled the disease until his death.

Peter mostly provided tenor vocals for the famous 1960s folk trio, but he also sang lead vocals for some of Peter, Paul and Mary's biggest hits ... most notably, "Puff The Magic Dragon."

He co-wrote 'Puff' in 1959 when he was studying at Cornell University ... and he also provided lead vocals on "The Great Mandala" and "Day Is Done," which cracked the Top 20 on the Billboard charts.

Peter even turned the timeless children's song into a picture book for kids.

The last time we saw Peter, he told our photog why 'Puff' was never a song about drug use and explained how the dragon got its famous name.

Peter's the second member of Peter, Paul and Mary to die ... Mary Travers died way back in September 2009, and now Paul Stookey is the last surviving member.

Peter was 86.

RIP