Gufran Ahmed Kauser Mohammed, 31, had pleaded not guilty in August when he was indicted, but changed his plea when he made an appearance before US District Judge Ursula Ungaro in Miami yesterday.
He will be sentenced on October 24 and faces up to 15 years in prison.
Mohammed was one of two men nabbed in the operation that saw an undercover FBI agent use an Internet chatroom to pretend to help finance Al-Qaeda and other militant groups in Syria and East Africa.
Mohammed was arrested in August in Dammam, Saudi Arabia, where he was living with his wife and daughter.
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According to the indictment, Mohammed made several transfers of funds to East Africa to help support Shebab fighters in Somalia, before making a transfer to the undercover agent, who was posing as a fundraiser, recruiter and supplier for Al-Nusra.
Mohammed and Said met in Saudi Arabia in May 2011 and Mohammed transferred more than $11,000 to Said from June to September that year, aiming to reach the Shebab.
The two defendants had also declared their support for Al-Qaeda and Al-Nusra, including by recruiting and moving experienced Shebab militants to Syria, according to the US Justice Department.