Saudi Arabia has listed the Muslim Brotherhood as a terrorist organisation along with two al-Qaeda-linked groups fighting in Syria.
The decree against the Brotherhood, whose Egyptian branch supported the deposed Egyptian president, Mohamed Morsi, was reported on Saudi state television on Friday.
Egypt in December listed the Brotherhood as a terrorist organisation, prompting the arrest of members and associates and forcing the Islamist group further underground.
Saudi Arabia also listed Jabhat al-Nusra, which is al-Qaeda's official Syrian affiliate, and the Islamic State of Iraq and the Levant (ISIL), which has been disowned al-Qaeda, as "terrorist organisations".
It also listed Shia Huthi rebels fighting in northern Yemen and the little-known internal Shia group, Hezbollah in the Hijaz.
It also ordered any citizen fighting abroad to return within 15 days or face imprisonment.
King Abdullah last month decreed prison terms of up to 20 years for belonging to "terrorist groups" and fighting abroad.
Similar sentences will be passed on those belonging to "extremist religious and ideological groups, or those classified as terrorist organisations, domestically, regionally and internationally," state news agency SPA said at the time.
Under the decree, supporting such groups, adopting their ideology or promoting them "through speech or writing" would also incur prison terms.