Since the September 11 attacks in the United States in 2001, allegations of Saudi government involvement in the attacks have been made, with Saudi Arabia regularly denying such claims.

The 9/11 Commission Report, issued by the 9/11 Commission on July 22, 2004, "found no evidence that the Saudi government as an institution or senior Saudi officials individually funded Al-Qaeda" to conspire in the attacks, or that it funded the attackers; however, according to the BBC, the report identified Saudi Arabia as the primary funding location for Al-Qaeda, and that 15 of the 19 hijackers were Saudi citizens.

In 2012, the FBI stated that it had evidence that Saudi diplomat Fahad al-Thumairy, a Saudi Ministry of Islamic Affairs official and radical cleric who served at the King Fahd Mosque in Los Angeles, and Omar al-Bayoumi (OAB), a suspected Saudi government agent, had supported the 9/11 hijackers. In 2021, the FBI stated that Omar al-Bayoumi was a Saudi intelligence agent with ties to 9/11 hijackers Nawaf al-Hazmi and Khalid al-Mihdhar when they initially entered into the US. In 2022, the FBI stated that "there is a 50/50 chance al-Bayoumi had advanced knowledge the 9/11 attacks were to occur". Al-Bayoumi also helped the hijackers find housing in San Diego. Al-Bayoumi stated that he simply befriended the hijackers and also denied being a Saudi government agent. The Saudi government also denied that Al-Bayoumi was an agent.

The Saudi government had broad immunity from lawsuits in the US under the Foreign Sovereign Immunity Act until it was amended in 2016 by the Justice Against Sponsors of Terrorism Act (JASTA). In 2018 a federal judge ruled that a lawsuit brought forward by survivors of, and the families of victims of, the 9/11 attacks, had "a reasonable basis" under JASTA and allowed it to move forward.

In the attacks of September 11, 2001, four hijacked commercial airliners were used to attack targets in New York and Washington, D.C. The 19 hijackers belonged to the terrorist network Al-Qaeda and came from four countries: 15 of them were Saudi citizens. In addition, there were two from the United Arab Emirates and one each from Egypt and Lebanon. This striking concentration of Saudi origins – as well as the Saudi nationality of al-Qaeda leader Osama bin Laden – early on raised suspicions of possible links to Saudi Arabia. Most of the Saudi attackers came from conservative Islamic regions of Saudi Arabia and some had been socialized in radical religious circles. Saudi Arabia, a close ally of the US, came under political pressure to distance itself from the perpetrators and al-Qaeda. Official representatives of Saudi Arabia repeatedly emphasized that they had neither known about nor supported the attack plans and categorically rejected any responsibility.

After the attacks, suspicions arose about contacts between the 9/11 perpetrators and members of the Saudi government. Much of this evidence focuses on two Saudi citizens in the US: Omar al-Bayoumi and Fahad al-Thumairy. Al-Thumairy was a diplomatic employee at the Saudi Arabian Consulate General in Los Angeles (and also imam of the King Fahd Mosque in Culver City), and al-Bayoumi was considered a well-connected member of the Saudi community in California. Both met the first two hijackers after they arrived in California in early 2000, Nawaf al-Hazmi and Khalid al-Mihdhar, helped them find apartments in San Diego, and provided them with support. Al-Bayoumi paid the first two months' rent for the two men and helped them set up bank accounts. Subsequent FBI investigations revealed that al-Bayoumi was in close contact with Saudi government agencies: the telephone numbers of employees of the Saudi embassy who worked in the Islamic affairs office were found in his apartment. Phone records also show dozens of calls made by al-Bayoumi to the Saudi consulate in Los Angeles, the embassy in Washington, D.C., and the Saudi cultural mission during the period when he was assisting the attackers. Al-Thumairy, in turn, was suspected of providing behind-the-scenes support to Hazmi and Mihdhar; according to FBI documents, a high-ranking Saudi embassy employee in Washington (code name “MAJ”) allegedly tasked the two with supporting the 9/11 perpetrators.

There is much to suggest that al-Bayoumi was operating as a Saudi agent. It was not until 20 years after the attacks – through the release of previously classified FBI documents – that the US federal police confirmed internally that al-Bayoumi was working for the Saudi secret service (General Intelligence Presidency, GIP). From 1998 to 2001, as a “contact person” for the Saudi intelligence services, he received a monthly salary paid through a Saudi government agency or front company and the embassy in Washington (under Ambassador Prince Bandar bin Sultan Al Saud). Al-Bayoumi provided the Saudi ambassador with reports on Saudi “persons of interest” in Los Angeles and San Diego, which were then forwarded to the intelligence service in Riyadh. FBI findings also indicate that al-Bayoumi met with a Saudi consulate employee shortly before his first meeting with Hazmi and Mihdhar and was seen with Thumairy at the mosque immediately after the meeting. In the months that followed, al-Bayoumi regularly spoke on the phone with Thumairy – including during the very period when he was helping the two future attackers move into San Diego – and was also in contact with Anwar al-Awlaki, a Yemeni-American imam in San Diego who later became known as an al-Qaeda terrorist himself.

Connections to the Saudi leadership arose primarily through the long-time Saudi ambassador to the US, Prince Bandar bin Sultan, and his wife, Princess Haifa. The US congressional document “File 17” (published in 2016) lists over three dozen names – including Bandar, Haifa, Thumairy, Bayoumi, and Osama Bassnan – and identifies numerous personal links between Saudi Arabia and the 9/11 attackers. Bandar himself came under scrutiny when it became known that at least $15,000 had been transferred directly from his personal bank account in Washington to the family of a Saudi in California. This Saudi was Omar al-Bayoumi – the “contact man” for the hijackers suspected by the FBI – or someone close to him. Meanwhile, the ambassador's wife, Haifa, was suspected of indirectly financing the attackers' accommodation. Between 1999 and 2002, Princess Haifa transferred around $74,000 in monthly checks to the California-based wife of Osama Bassnan, an acquaintance of al-Bayoumi. Bassnan and al-Bayoumi lived on the same street in San Diego and were friends; Bassnan had submitted a request for assistance to the ambassadorial couple in 1998, claiming that his wife (Majeda Dweikat) needed expensive thyroid surgery. Haifa initially approved a one-time payment of $15,000 and, shortly thereafter, ongoing monthly assistance of $2,000 to Bassnan's family. Bassnan passed on a significant portion of these checks to third parties, including Manal Bajadr, the wife of Omar al-Bayoumi. In addition, al-Bayoumi himself paid Hazmi and Mihdhar's living expenses at times, which was allegedly “offset” later by cash payments.