According to the official Saudi news agency on Monday, Yasir Isam Hamdi left US custody in South Carolina and was flown by military aircraft to Dammam airport in the Kingdom's eastern province.
Hamdi had allegedly fought with the Taliban in Afghanistan but no specific charges were ever made against him nor did he ever appear in any court.
Held in isolation for nearly three years, he renounced his American citizenship immediately on arrival as part of a complicated release deal, the interior ministry said.
US officials have also imposed stringent restrictions on his travel and said he must report to the US embassy in Riyadh if he ever wants to leave the country.
No more prison
A security source said Hamdi was "not wanted by Saudi authorities" and no security-related charges had been made against him. But family members said they still expected him to be interrogated by authorities to "clear up his position".
John Lindh was known as the
American Taliban in Afghanistan
Born in the US state of Louisiana but later becoming a Saudi citizen, Hamdi was one of three US nationals known to be held as enemy combatants under the US "war on terror".
John Walker Lindh, the so-called American Taliban, is serving a 20-year jail term. Jose Padilla, who was detained in May 2002, is still in detention, accused of planning a radiological "dirty" bomb attack on the US.