The police officer's defense claimed the incident was an involuntary gesture against an agitated person.
However the court in the Northern city of Boulogne-sur-Mer saw the incident which took place in a detention center near Calais in the summer of 2016 differently, handing him a six-month prison sentence.
The 40-year-old officer, who was a member of France's border police (PAF), slapped the migrant just before he was due to appear before a judge in a trial room at a detention center in Coquelles.
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The officer slapped the migrant in the presence of a court clerk who then testified against him.
However his defense presented a somewhat different version of the story.
“It was an unfortunate act of protection, he did not want to slap,” said his lawyer Antoine Deguines. “This person was quite agitated, and was getting up all the time, and on the spur of the moment he [the officer] stretched his arm which went in the person's face.”
The judgement against the officer, who has also been banned from holding a weapon for five years and suspended from performing public office for two years, comes at a time when the treatment of migrants and refugees by French border police is under scrutiny.
A report conducted by the French state and released at the end of October said that it was “plausible” that police and gendarmes have abused their powers and mistreated migrants and refugees in Calais.
And in July, rights group Human Rights Watch blasted French police for their routine use of pepper spray against migrants in Calais in a report entitled “Like living in hell”.
Of 61 migrants questioned by the prominent international human rights group between the end of June and the beginning of July, 55 said they had been sprayed during the two weeks before the interview, and some said they had been sprayed every day, the author of the report, Michael Garcia Bochenek, told AFP at the time
French police have also been accused of spraying migrants' food and bedding and confiscating blankets, shoes and food.