The aid, from organisations including the World Bank, African Development Bank (AfDB), European Union and Islamic Development Bank (IDB), will support efforts to boost economies and stem conflict and hunger across the volatile region.
Countries targeted are Djibouti, Eritrea, Ethiopia, Kenya, Somalia, South Sudan, Sudan, and Uganda.
"The countries of the Horn of Africa are making important yet unheralded progress in economic growth and political stability," Ban said in a statement.
Ban, who begins his Horn of Africa trip in Ethiopia today, is due to travel onwards to neighbouring Djibouti and Kenya, leading a delegation from six other international organisations.
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Alongside Ban is World Bank president Jim Yong Kim, and officials from the African Union, EU, the regional Intergovernmental Authority on Development (IGAD) bloc of nations, as well as the AfDB and IDB.
Projects include support for oil pipelines and improving transport links, as well as boosting education and internet access.
While in Nairobi, Ban will also launch a global campaign to end female genital mutilation.