The senior Telugu Desam Party (TDP) leader was found lying unconscious at his posh Banjara Hills residence at around 11 a.m. by family members.
His daughter with the help of driver and security guard shifted him to Basvatarakam Indo-American Cancer Hospital, where doctors declared him dead on arrival.
The police has registered a case of death under suspicious circumstances and launched investigations.
Deputy Commissioner of Police A.R. Srinivas said the cause of death would be known after the autopsy report. The family members told the police that he was under mental stress.
Rao's body has been shifted to the government-run Osmania General Hospital for autopsy.
Opposition TDP blamed the YSR Congress Party (YSRCP) government for Rao's death.
A physician by profession, Rao entered politics at the invitation of TDP founder N.T. Rama Rao in 1982. A senior leader from Guntur district, Siva Prasada Rao served as a Minister in the cabinets of N.T. Rama Rao and N. Chandrababu Naidu in undivided Andhra Pradesh.
He served as Assembly Speaker from 2014 to 2019. He was defeated in the recent Assembly polls.
Siva Prasada Rao, a six time MLA, is survived by wife, two sons and a daughter.
A TDP strongman, he was blamed by his rivals for political violence in Guntur's Palnadu region.
A bomb blast at his house in Narsaraopet during the 1999 elections killed four of his supporters. He was then a Minister in Naidu cabinet.
The Congress party had accused him of storing bombs at his house but Siva Prasada Rao blamed Maoists for the blast.
The Central Bureau of Investigation (CBI) had later him a given clean chit in the case.
In April, Siva Prasada Rao was injured in an alleged attack by YSRCP workers at a polling station. The YSRCP had accused him of resorting to rigging.
Siva Prasada Rao and his son Siva Rama Krishna were last month booked by the Andhra Pradesh Police for allegedly diverting the Assembly furniture to unauthorized premises. Some furniture was recovered from Krishna's motorbike showroom in Guntur.
As the news of his death spread, several leaders of TDP and Telangana Rashtra Samithi (TRS) rushed to the hospital. They expressed their shock over his sudden death.
Andhra Pradesh Chief Minister Y.S. Jagan Mohan Reddy expressed grief and conveyed his condolences to the bereaved family members.
Telangana Chief Minister K. Chandrashekhar Rao expressed shock over the former Assembly Speaker's demise.
TDP president and former Andhra Pradesh chief minister N. Chandrababu Naidu said Siva Prasada Rao's death was a huge loss to the party.
( With inputs from IANS )