Cédric Herrou was sentenced by an appeal court in Aix-en-Provence, near Marseille in the south of France.
He has housed dozens of migrants in caravans on his farm in the Roya valley, in south-east France.
In February he received a suspended 3,000 euro (£2,700) fine.
Prosecutors had sought an eight-month suspended jail term for Herrou, for helping migrants to slip past police after entering from Italy.
Outside the court, Herrou told reporters: "It's the role of a citizen in a democracy to act when the state is failing."
"I'd like the judiciary to recognise what's happening on the ground in the Roya valley, recognise these asylum seekers. What am I to do, really? Kick these people out?" he added.
He said he had "no regrets" and "I won't be stopped by threats - quite the opposite".
Herrou has become a symbol of ordinary Europeans who have taken action to aid migrants fleeing war or poverty in the Middle East and Africa.
He has housed teenagers from Eritrea and Sudan on his property, reports say.
Last month French President Emmanuel Macron described a plan to set up "hotspots" in Libya to process asylum seekers, because so many migrants were still "taking crazy risks" crossing the Mediterranean.
He said nearly a million migrants were currently in camps in Libya, hoping to reach Europe, but were not eligible for asylum.
Most of the migrants rescued from overcrowded boats and brought to Italy this year have been sub-Saharan Africans. Many have suffered violence, including rape, but they do not qualify for asylum if they are not victims of persecution.
The UN refugee agency UNHCR says nearly 118,000 have reached southern Europe by sea so far this year, more than 96,500 of whom came ashore in Italy.
A note on terminology: The BBC uses the term migrant to refer to all people on the move who have yet to complete the legal process of claiming asylum. This group includes people fleeing war-torn countries such as Syria, who are likely to be granted refugee status, as well as people who are seeking jobs and better lives, who governments are likely to rule are economic migrants.
A French farmer who helped migrants to enter France illegally has been given a four-month suspended jail sentence, after prosecutors argued that an earlier suspended fine was too lenient.