A suicide bomber has killed four security officers and wounded five others near the security headquarters of the Prophet's Mosque in Medina.
The blast in the city - the second-holiest site in Islam - followed three explosions in Qatif and Jeddah on Monday on the penultimate day of the Muslim holy fasting month of Ramadan.
The Al Arabiya news channel showed a fire burning in a car park near the mosque in Medina.
Four security officials were killed, according to the Saudi Interior Ministry.
A witness said a car bomb was detonated near a mosque in Qatif and was followed by a suicide bomb attack just before 7pm local time.
Qatif is heavily populated by Shia Muslims, who are a minority in the Sunni-ruled kingdom.
A suicide bomber had earlier set off a device near the US consulate in Jeddah, wounding two security officers.
Interior Ministry spokesman Major General Mansour al Turki said the attacker was spotted by security guards, and set off his device when he was approached.
No claims of responsibility for any of the attacks have been made.
Islamic State has carried out a series of bombing and shooting attacks in Saudi Arabia since 2014 that have killed scores of people, mostly Shias and members of the security services.
Most of the attacks have taken place in Eastern Province, which is home to the majority of Shias in the Gulf state.
In January, a suicide bomber attacked a mosque in al Ahsa, killing four people before worshippers disarmed and tied up an accomplice who had shot at them.
In October last year, a gunman opened fire on worshippers marking the Shia commemoration of Ashura in Qatif, killing five people before he was shot dead by police.