Thursday's ruling is the first conviction related to alleged abuses at the country's mines.
The High Court found Joseph Chani (51), a former police chief superintendent, guilty of murdering the illegal diamond panner in Zimbabwe's Marange fields and the assault of three others.
Prosecutors said Chani, who retired from the police force during his trial, beat Tsorotsai Kusena (37) with clubs, leading to his death.
Judge Hlekani Mwayera said the police officer would also serve three years concurrently for the assault of the three other panners.
Junior police officers and soldiers who guard the fields gave evidence during the trial about how Chani had assaulted the small-scale miners.
The Marange fields have been at the centre of a years-long controversy over alleged abuses by President Robert Mugabe's army, and the Kimberley Process once suspended exports from there.
But last year the international watchdog cleared Zimbabwe to export from Marange, the site of one of Africa's biggest diamond finds in recent years.
Rights groups have alleged gross human rights violations in the Marange fields, when the Zimbabwean army cleared small-scale miners from the area in late 2008.
Human Rights Watch says more than 200 people were killed in the operation. – AFP