Khalil Rasjed Dale was managing a health program in the city of Quetta in southwestern Pakistan when gunmen seized him earlier this year.
A note attached to his body said he was killed because a ransom wasn't paid.
The identities of his captors are not known, but the region is home to militants who have kidnapped for ransom before.
British Foreign Secretary William Hague condemned Dale's killing, and said "tireless efforts" had been under way to secure his release after he was kidnapped.
Khalil had worked for the Red Cross for years, carrying out assignments in Somalia, Afghanistan and Iraq, the group said.
The Associated Press contributed to this report.