"Banharn Silpa-archa passed away peacefully," said a statement from Siriraj hospital, where he was admitted with asthma on Thursday.
Born to a Chinese family in central Suphan Buri province in 1932, Banharn was first elected an MP in the seventies and went on to become Thailand's 21st prime minister in 1995.
He stepped down the following year following a string of scandals and a bruising no-confidence debate in parliament.
His Chart Thai party nevertheless remained a key coalition partner in governments led by then-prime minister Thaksin Shinawatra and the rival Democrat Party, earning the nickname "eel" for its slippery alliances.
But he continued to wield influence as an advisor to the party after it was reincarnated under a different name.
Banharn, who also managed a handful of ministerial portfolios in the 1990s, is survived by three children, two of whom have also led careers in government.