Vitalmiro Bastos de Moura was convicted of masterminding the killing of 73-year-old Dorothy Stang, on Feb. 12, 2005, along a muddy stretch of road deep in the rain forest.
Judge Raymond Moises Alves Flexa sentenced Moura to 30 years in prison, the maximum sentence, in a case seen as a test of whether the government could crack down on lawlessness in the Amazon.
Moura "showed a violent personality unsuited to living in society," the judge said, adding that the "killing was carried out in violent and cowardly manner."
The conviction came even though three other men convicted in connection with the killing — a gunman, his accomplice and a go-between — recanted earlier testimony that the rancher had offered them 50,000 reals (US$25,000; euro18,400) to kill the nun.
Moura is one of two ranchers accused of ordering Stang's killing in a conflict over land he wanted to log and develop but she wasn't trying to protect.
Stang, a naturalized Brazilian originally from Dayton, Ohio, helped build schools and was among the activists who have tried to defend the rights of impoverished and often exploited farmers drawn to the Amazon region. She also attempted to halt the rampant jungle clearing by loggers and ranchers that has already ripped away some 20 percent of the forest cover.
Human rights defenders said the trial is a test of whether the powerful masterminds behind land-related killings can be held accountable in the Amazon state of Para. Of nearly 800 such killings in Para during the past 30 years, only four masterminds have been convicted and none are behind bars.
Shortly after Stang's killing, President Luiz Inacio Lula da Silva ordered the army into the region, suspended logging permits, and ordered large swathes of rain forest off-limits to development.