Daria Morozova, the human rights ombudsperson for the Moscow-backed separatist leadership in Donetsk, said Paul Urey, who was a British “mercenary”, died in captivity on Sunday.
She said he had died of chronic illnesses and stress.
“From our side, he was given the necessary medical assistance despite the grave crimes he committed,” she added.
The Presidium Network, a non-profit group, said in April that Mr Urey and Dylan Healey were captured at a checkpoint south of the city of Zaporizhzhia in south-eastern Ukraine.
Mr Urey, who was born in 1977 and is from Manchester, and Mr Healey, born in 2000 and from Cambridgeshire, travelled to Ukraine of their own accord, the organisation said.
They were not working for the Presidium Network, which helps to get aid into Kyiv.
The organisation said the pair were driving to help a woman and two children to evacuate when they went missing.
Presidium Network said it was concerned Russian forces may think the two men are British spies.
The Foreign Office said at the time that it was urgently seeking more information following reports of British nationals being detained in Ukraine.