Thursday's announcement notes that al-Hijra was formed in Nairobi in 2008.
The group "consists primarily of Kenyan and Somali followers of al-Shabaab in East Africa," the statement added.
The Kenya-based wing of Al-Shabaab, known as Al-Hijra, was added on Thursday to the US list of groups it regards as terrorist organisations.
"Terrorism designations expose and isolate organisations and individuals, and deny them access to the US financial system," the State Department said in explaining the move.
"Moreover, designations can assist the law enforcement activities of US agencies and other governments."
Thursday's announcement notes that Al-Hijra was formed in Nairobi in 2008.
The group "consists primarily of Kenyan and Somali followers of Al-Shabaab in East Africa," the statement added.
"It has openly engaged in Al-Shabaab recruiting in Kenya and facilitated travel of Al-Shabaab members to Somalia for terrorism purposes."
Kenyans affiliated with Al-Hijra -- and consequently with Al-Shabaab -- may have played a role in major terrorist attacks inside Kenya in recent years, including the Westgate Mall massacre in 2013 and the attack on Garissa University College in 2015.
The State Department also announced on Thursday that it is extending its designation of Shabaab itself as a global terrorist organisation following completion of a mandated five-year review of the group's operations.
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