John Williams Ntwali, 44, editor of The Chronicles newspaper, died on January 18 when a vehicle rammed into the motorcycle he was riding as a passenger near the capital Kigali.
The driver involved in the crash was convicted of "involuntary manslaughter" and ordered to pay a fine of one million Rwandan francs, about 860 euros.
Reporters Without Borders, which goes by its French initials RSF, said in a statement that the trial was a "blitz" and "an opaque procedure that left many areas of shadow over the circumstances of his death".
"This judgment was made on the basis of an investigation that was never made public and which obviously only took into account the hypothesis of an accident, without exploring other avenues," it said.
"In 2012, the journalist, who was already the subject of threats, was the victim of a similar accident which he survived."
"Ntwali", as many called him, had been imprisoned numerous times during his career -- sometimes for hours, sometimes for weeks.
He founded the Pax TV channel on YouTube, mostly broadcasting interviews in the Kinyarwanda language with dissident figures.
In the past decade, independent media has dwindled and been muzzled by the government.
Praised for the successes of his development policy, President Paul Kagame has also been criticized by rights groups for his crackdown on free speech.
"Since 1996, eight professionals have been killed or are missing, and 35 have been forced into exile," RSF said on its website, adding that Rwanda ranked 136th out of 180 countries for press freedom.