A German nurse working for the International Committee of the Red Cross (ICRC) was kidnapped in Somalias capital Mogadishu, the aid group said.
She was seized by unknown armed men from inside the ICRC compound at 20:00 local time (17:00 GMT) on Wednesday.
The 10 remaining international staff are being evacuated to Nairobi until further notice, the ICRC told the BBC.
A Somali woman working for the World Health Organization, also in Mogadishu, was shot dead by gunmen on Tuesday.
A relative of the woman, identified as Maryan Abdullahi, said she was targeted while at a market to buy items in preparation for her wedding next week.
There is no suggestion her killing and the abduction are linked.
Decades of conflict have made Somalia one of the worlds most dangerous countries for aid workers, but security has improved in recent years and Wednesdays kidnapping is the first involving a foreign worker in almost 10 years.
"We are deeply concerned about the safety of our colleague," said Daniel OMalley, the ICRCs deputy head of delegation for Somalia, in a statement.
"She is a nurse who was working every day to save lives and improve the health of some of Somalias most vulnerable people."
Staff believe the kidnappers took their colleague out through a back door, avoiding security guards stationed at the main entrance of the ICRC compound, Reuters reported.
A police hunt for the nurse is under way. It is not known who is behind the kidnapping.
The Swiss-based agency, which has provided humanitarian aid in Somalia for years, has temporarily suspended movement in Mogadishu but said Somali staff would continue ICRCs programmes outside the capital.
Somalia collapsed into anarchy in 1991 when war lords overthrew a military regime. Major armed conflict has since subsided, which allowed for parliamentary and presidential elections to take place in 2012.
However, al Qaeda-linked al-Shabab militants still regularly launch attacks in the capital and elsewhere in the country.