The 55-year-old woman, whose nationality was not disclosed, was suffering from chronic illnesses, a statement said.
The health ministry said five other people living in Riyadh were infected with the Middle East Respiratory Syndrome, two of them foreigners.
Late on Thursday, the ministry reported the death of a 70-year-old Saudi woman in the western city of Jeddah where the virus has spread in recent weeks.
It reported six other infections in Jeddah, among them an expat "health worker," bringing to 218 the number of MERS infections in the worst-hit country.
Panic over the spread of MERS among medical staff in Jeddah this month forced the temporary closure of an emergency room at a major hospital, prompting Health Minister Abdullah al-Rabiah to visit the facility in a bid to calm the public.
Local media reported Wednesday at least four doctors at Jeddah's King Fahd hospital have resigned after refusing to treat patients affected by MERS, apparently out of fear of catching the virus.
The MERS virus was initially concentrated in the eastern region of Saudi Arabia but has now spread across other areas.
The World Health Organisation said Thursday it had been told of 243 laboratory-confirmed cases of MERS infection worldwide, of which 93 have proved fatal.
Malaysia announced the first MERS death Sunday in the country of a man who had developed a fever, cough and breathing difficulties after returning from a pilgrimage to Saudi Arabia's holy city of Mecca on March 29.
The case prompted Malaysian authorities to quarantine 64 people in the dead man's village.
The MERS virus is considered a deadlier but less-transmissible cousin of the SARS virus that erupted in Asia in 2003 and infected 8,273 people, nine percent of whom died.
Experts are still struggling to understand MERS, for which there is no known vaccine.
A recent study said the virus has been "extraordinarily common" in camels for at least 20 years, and may have been passed directly from the animals to humans.