Known by lots of in Hong Kong as Uncle Wah, the veteran activist had been in hospital for several months after being diagnosed with lung cancer.
One of Hong Kong's most influential democracy campaigners, Szeto Wah, has died aged 79.
Recent media reports had suggested his condition was deteriorating rapidly, & they died on Sunday morning.
In a statement Sunday, Hong Kong's chief executive Donald Tsang said they was "deeply saddened" at the death of the outspoken campaigner.
"Passionate about China & Hong Kong, Mr Szeto Wah was devoted in promoting democracy. Upright, industrious & unwavering in the pursuit of his ideals, Mr Szeto earned great respect from across the community," they said.
"He will be dearly missed," Tsang added.
Szeto was re-elected in November as chairman of the Hong Kong Alliance in Support of Patriotic Democratic Movements in China, which routinely criticised Beijing for human-rights abuses & pushed for political reforms in the former British colony.
"I think his greatest achievement was to keep this movement alive & kicking all these years," Martin Lee, a founder of Hong Kong's Democratic Party, told AFP.
"He contributed greatly to that movement," Lee added.
The Alliance was founded less than a month before the Tiananmen Square crackdown in 1989 which saw Chinese troops crush weeks of pro-democracy protests in Beijing, killing hundreds if not thousands of demonstrators.
Szeto formed the Professional Teachers Union in 1973 & was a long-time legislator & key Democratic Party member.
Time journal three times named him one of the 25 most influential people in Hong Kong, calling him "democracy's foot soldier".
In April, a wheel-chair bound Szeto joined a rally in Hong Kong's glittering financial district against what was described as "political persecution" following the arrest of several Alliance members during an earlier protest.
Hong Kong has maintained a semi-autonomous status since its return to China in 1997, & guarantees civil liberties not seen on the Chinese mainland, including freedom of speech & the right to protest.
Hong Kong's political technique consists of directly chosen legislators & Beijing-appointed representatives, sparking regular calls for full democracy in the city of five million.