Security forces stopped the teenagers yesterday as they tried to enter Morocco allegedly to join Islamic State, whose fighters have seized swathes of Iraq and Syria, the government said.
"The detention of two women recruited for jihad is a remarkable and unprecedented event in Spain," the interior ministry said in a statement.
The girl, who could not be identified because she is a minor, was just 14, an official said. The 19-year-old woman was named as Fauzia Allal Mohamed. Both are Spanish citizens.
The pair were detained at the Beni Enzar border crossing in Melilla, one of two tiny Spanish territories on the north African coast. Both Melilla and the other Spanish city, Ceuta, share a border with Morocco.
"Both were trying to cross the border to Morocco with the aim of contacting the network which would move them immediately to a conflict zone between Syria and Iraq," the Spanish ministry said.
"Their intention was to join one of the cells of the terrorist organisation of the self-proclaimed Islamic State," it said.
"The two women detained in this police operation are a clear example of this," the interior ministry said.
"Their radicalisation, recruitment and later dispatch as combatants were perfectly planned and organised by a network that operates across north Africa and has as its main goal getting the maximum number of unquestioning combatants.