"In France today we have the right to save people in distress," said researcher Pierre-Alain Mannoni, who had faced a six-month suspended jail sentence for aiding Eritrean migrants who came into France from Italy.
The prosecutor said during the trial in November in the southern city of Nice that people had a "duty" to help people, "but not help (illegal immigrants) to stay and circulate" in the country.
The judge ruled Friday that the 45-year-old researcher at French national research centre CNRS had helped three young Eritrean women to "protect their dignity".
A law in effect since 2012 grants immunity to people smugglers who are not paid and if the lives of their charges are considered in peril.
"You can help people," Mannoni said after the ruling. "I recommend it, it does a lot of good!"
He is one of three people in the area to appear in court recently for illegally assisting migrants travelling up through Europe after crossing the Mediterranean in rickety boats.
Their cases have pitched the spirit of solidarity against the letter of the law at a time when border controls and migration have become hot political issues.
The right-wing president of the Riviera region, Christian Estrosi, has repeatedly criticised activists who help migrants.
He said on Facebook that Friday's ruling was "an insult to the work of the security forces who put their life in danger to protect ours."
Estrosi added that some jihadists had been able to enter France after passing themselves off as migrants.
Mannoni was arrested at a toll booth near the southern town of Menton in October with the three Eritreans including a minor that he wanted to lodge. Another activist, farmer Cedric Herrou, appeared in the Nice court on Wednesday facing an eight-month suspended prison term for moving some 50 Eritreans into a disused holiday village owned by the state.
Security forces emptied the camp after three days.
In addition to the suspended sentence, public prosecutor Jean-Michel Pretre called for Herrou's vehicle to be confiscated and for his driving licence to be restricted to driving only for professional use.
Herrou, 37, has become something of a folk hero around the French-Italian border for driving migrants across the frontier, under the noses of the French police, and then putting them up. The verdict in his case is expected on February 10.
In early December an appeals court in Aix-en-Provence upheld a fine of 1,500 euros ($1,580) against a 73-year-old retired professor for helping two Eritreans avoid a police check.
The controversy comes less than four months ahead of the first round of France's presidential elections.
The Socialist government came under harsh criticism for allowing the infamous "Jungle" migrant camp to develop near the northern port of Calais, which the authorities finally closed in October.