The 86-year-old Abdullah flew to the United States on Monday but it was not immediately clear which US state Abdullah has headed for.
King Abdullah reportedly suffers from a herniated disc and blood accumulation around the spine.
The Saudi king since 2005, Abdullah reduced his duties in June over health problems never previously officially disclosed.
On Sunday, Saudi Arabia's Health Minister Abdullah al-Rabeeah said that the monarch was "in a stable state and is well," and that he will receive further medical examination and continued treatment at a center specializing in spinal problems in the US.
In the king's absence, his half brother Crown Prince Sultan bin Abdul Aziz is running the country.
The 85-year-old Sultan, who has suffered his own serious health problems, arrived earlier in the day after a three-month stay in Morocco.
The aged crown prince is now left with control of the Mideast powerhouse which tops the list of the world's oil producers and is seen as the key US ally and energy supplier.
Sultan has been receiving treatment for the past two years but Riyadh has not officially announced his problem. Some analysts and diplomats, however, believe the Saudi prince suffers from cancer.
Abdullah and Sultan are not the only members of Saudi Arabia's aged royals to be treated for health problems during the past year.
The kingdom's 76-year-old Prince Nayef has also had unspecified treatment in the past months.
Nayef, Sultan's full brother and considered second in line to the throne, has held the post of interior minister for the past 35 years.
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