Sergei Bagapsh, president of Georgia's breakaway region of Abkhazia, has died aged 62, Abkhaz officials say.
They say he passed away in a Moscow hospital from complications following recent lung surgery.
Mr Bagapsh won his second five-year term in 2009. Georgia denounced the elections as "illegitimate".
Since its brief war with Georgia in 2008 over another disputed territory, South Ossetia, Russia has provided security for Abkhazia.
Abkhazia declared independence from Georgia in 1992, but is only recognised by Moscow and a handful of other countries.
Abkhaz officials say Vice-President Alexander Ankvab will be acting leader.
Russia sent forces into the two breakaway regions in a brief war in August 2008.
Thousands of ethnic Georgians fled the regions during the conflict and many remain internally displaced in Georgia.
In 2008, Western countries condemned Russia for penetrating deep into Georgia, beyond the conflict zones.
Russia later pulled its forces back to the two regions.
In September 2009 an EU-sponsored report said both sides had violated international law. It found that Georgia had attacked the Russian-backed South Ossetian rebels, triggering the war, after months of provocation.