At a Riyadh palace, Islamic clerics, tribal chiefs and prominent national figures took part in the Islamic tradition known as bayaa, shaking the king's hand and pledging allegiance.
Before the ceremony, British Prince Charles met with Abdullah to express condolences for the death of his half-brother, King Fahd on Monday. U.S. Vice President Dick Cheney, former President George H.W. Bush and French President Jacques Chirac were to meet the new king later in the day, the BBC said.
As crown prince, Abdullah bin Abdul-Aziz has effectively been the ruler for the last 10 years, since King Fahd suffered a debilitating stroke in 1995.
Fahd was buried Tuesday in an unmarked plot in a public cemetery in line with Saudi custom.