Hiendz Shutau, a resident of Brest in southwest Belarus, died in a military hospital in the capital Minsk, according to the TUT.by website.
The 43-year-old was shot in the head on August 11 when authorities dispersed peaceful mass protests against the presidential election results, according to media reports.
He was treated by medics in Brest and then transported to Minsk.
Other sources say a total of four protesters have now died in Belarus.
European Union leaders were on Wednesday meeting to discuss Belarus, where protests are mounting against Lukashenko.
The presidents of Poland and three regional partners on Wednesday called on the Belarus authorities to respect human rights and refrain from using violence against peaceful demonstrators.
Tens of thousands have taken to the streets in Belarus to demonstrate against the election, which protesters say was rigged, and against police brutality towards citizens.