It is the third time Vitalmiro Bastos Moura has been tried for Dorothy Stang's 2005 death after previous convictions were overturned.
Ms Stang, 73, campaigned for 30 years to save rainforest from the interests of wealthy landlords.
She had tried to block Moura's attempts to seize land and was shot six times.
Moura, who was convicted in 2007 and 2010 in verdicts that were subsequently overturned, was sentenced to 30 years in prison.
In the latest trial, the judge said Moura had acted in a cold-blooded and cowardly manner.
Prosecutors earlier said Moura and another rancher had hired gunmen to kill Ms Stang. Moura's lawyers argued there wasn't enough evidence linking him to the crime.
The death of Ms Stang in the northern state of Para became a symbol of the violent conflict for natural resources in Brazil's Amazon region.
In the past 30 years, more than 1,000 people have been killed in land disputes in Brazil, with some two-thirds of the murders happening in Para.
Ahead of the latest verdict, Ms Stang's sister, Barbara Richardson, said the repeated trials had been upsetting but had helped maintain a spotlight on the issues.
"That keeps the injustice of the system before the people's eyes," she told AP. "Dorothy is ever prominent. The people understand what happened to her and what this means."

16 January 2017 Last updated at 17:18 GMT
Mr Murphy said the party had been "stretched to the limits" by their former partners in government, the DUP. (Democratic Unionist Party).

Two of the team's stars, Sir Bobby Charlton and Sir Geoff Hurst, recounted memories at Wembley Arena as the crowd of 10,000 relived the historic day.
Actors including Martin Freeman read the words of fans and players, including the late Sir Bobby Moore.
There was also live music from performers including Sophie Ellis-Bextor, Reef and the Troggs.
Chris Farlowe gave a one-off performance of his 1966 hit Out of Time - which was number one in the charts half a century ago.
The audience heard fans' memories of that July Saturday, including stories of weddings where men wore earpieces connected to radios so they could listen to the commentary.
The match ended with Alf Ramsey's England side beating West Germany 4-2 in extra time.
It was a day when the names of Banks, Cohen, Wilson, Charlton J, Moore, Ball, Stiles, Peters, Charlton R, Hunt and Hurst earned their place in football history.
The hashtag #worldcup1966 is trending on Twitter, and the Foreign Office has tweeted to say the UK's ambassador to Azerbaijan has met the family of Tofiq Bahramov - the Azerbaijani linesman who awarded Sir Geoff's second goal.
But a tweet from actor Gary Kemp shows not everyone was paying close attention at the time.
"Six years old and bored of my family watching the match I went out on my own to the empty park and sat on a swing. #worldcup1966," he tweeted.
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Bobby Moore and Alan Ball have since died, in 1993 and 2007 respectively. A bronze statue of Moore stands outside Wembley Stadium and is likely to be visited by many of fans attending the commemoration at the nearby arena.
At 21:00 BST, ITV will broadcast a documentary called 1966 - A Nation Remembers.
A Brazilian court has convicted a rancher for ordering the murder of an American nun over a land dispute - a case that caused international outrage.
[NEXT_CONCEPT]
Sinn Féin's Conor Murphy says it's time for an election as the deadline passes for Secretary of State James Brokenshire to call one, after the party refused to re-nominate a deputy first minister.
[NEXT_CONCEPT]
England's 1966 World Cup win has been marked by a special Wembley event exactly 50 years on.