Doctors Without Borders (MSF) resumed its work in a hospital in Yemen's second city of Aden on Sunday after a one-month suspension following the kidnapping and killing of a patient.
MSF stopped admitting people early last month when a group of armed men threatened staff and took a patient who was later found dead on a street.
"MSF Aden Trauma hospital has resumed its activities as of today May 5th, after a month of suspension following a security incident that happened in the hospital in which a patient was taken from the hospital and found killed nearby," it said on Twitter.
The southern port city of Aden has served as the seat of Yemen's beleaguered government since early 2015, when President Abedrabbo Mansour Hadi fled the capital Sanaa in the face of a rebel takeover.
A Saudi-led military coalition intervened in support of the government as the Houthi rebels closed in on Aden and Hadi went into exile.
Many of the country's hospitals have been damaged or destroyed and the country is heavily dependent on aid groups for medical care.
In November, MSF announced it had suspended its work in Daleh, a region inland from the Red Sea port of Hodeida, in the face of multiple security incidents directly targeting patients and staff.
The more than four-year conflict in Yemen has killed tens of thousands people, many of them civilians, relief agencies say.
The fighting has triggered what the United Nations describes as the world's worst humanitarian crisis, with 3.3 million people still displaced and 24.1 million -- more than two-thirds of the population -- in need of aid.