The body of Khalil Rasjed Dale was discovered on a road outside the city in southern Baluchistan province with a note saying he had been killed because a ransom had not been paid.
Dale, who had been working for the International Committee of the Red Cross (ICRC), was kidnapped in January while driving near the organisation's Quetta office.
"We are devastated," said the ICRC director general Yves Daccord. ''Khalil was a trusted and very experienced Red Cross staff member who significantly contributed to the humanitarian cause.
"All of us at the ICRC and at the British Red Cross share the grief and outrage of Khalil's family and friends."
The 60-year-old Muslim convert was a longstanding ICRC and British Red Cross worker. He had been managing an ICRC health programme in Quetta for almost a year.
The ICRC operates in some of the world's most dangerous places by trying to remain neutral and meeting with all sides of conflicts.
Both separatist militants and the Taliban are extremely active in Quetta, which is just a couple of hours' drive to the border with Afghanistan's Kandahar province, where the Taliban is battling US forces.
But while the organisation has working relations with movements such as the Taliban, its staff remain vulnerable to criminals and kidnappers.
William Hague, the foreign secretary, said "tireless efforts" had been made to secure Dale's release and the British government had worked closely with the Red Cross.
"I utterly condemn the kidnapping and killing of Mr Dale, and send my deepest condolences to his family and loved ones as they come to terms with their tragic and distressing loss," he said.
