Shaun Pinner, from Bedfordshire, was sentenced to death by a Russian-backed separatist court in eastern Ukraine after fighting for Kiev in the war with Moscow.
Now, the Russian state news agency Tass reports that he has appealed his sentence, the Telegraph reports.
Moroccan fighter Brahim Saadoun has also appealed the decision.
TASS said the Supreme Court of the self-proclaimed Donetsk People's Republic (DPR) had received complaints from lawyers for both.
It was reported that another convicted Briton, Aidan Aslin, had not yet lodged an appeal, citing Aslin's lawyer.