Bandic felt unwell around midnight Saturday and was swiftly taken to a hospital but nothing could be done, the state HRT television reported.
The populist Bandic was one of Croatia’s best-known politicians running Zagreb for the past 21 years amid a series of corruption scandals.
Croatia's anti-graft authorities have launched several investigations against Bandic, who was detained in 2014 but was still later re-elected to the post.
Mourning his death, some citizens lit candles Sunday outside Bandic's Zagreb home.
Bandic “lived for his city and its citizens, who always came first, and for his job, which he did with energy and love, and to which he gave the whole of himself,” Bandic's office said.
Bandic has had previous health problems.
Funeral arrangements are yet to be announced. Bandic left behind a wife and a daughter.