Vitaly Shishov's body was found hanged in a park in Kiev, a day after he failed to return from a jog, the British public broadcaster reported on its website.
It said police were investigating whether Shishov had been killed and his death made to look like suicide.
Shishov led the Belarusian House in Ukraine, helping those who left Belarus after a violent crackdown, the BBC reported.
It noted that Ukraine, Poland and Lithuania in particular became favoured destinations for those fleeing persecution following a disputed presidential election in Belarus in August last year.
Friends of Shishov reported that he had been followed recently, the BBC also reported, citing a Belarusian journalist named Tadeusz Gizcan.
The Belarusian House in Ukraine says it aims to provide support to new arrivals in finding accommodation, jobs and legal advice.