Abdellatif Merdas was gunned down late Tuesday near his home in the well-off southern district of Californie.
The 53-year-old was in his official car when he was hit three times by shotgun fire, private news websites reported. Hit in the head, he succumbed to his injuries shortly afterwards.
Local news websites showed photos of the scene and a car with broken windows, surrounded by police officers. They said one or more attackers fled in a foreign-registered vehicle.
Protection of non-Muslims summit to be held in Morocco
Moroccan police said on Wednesday that a suspect had been arrested.
"This individual with a criminal record is suspected of an alleged link to the armed homicide that killed a Member of Parliament Tuesday night," said a statement carried by the official MAP agency.
Merdas was a member of the liberal Constitutional Union (UC) party, representing the Ben Ahmed district 60 kilometres south of Casablanca.
Shootings are rare in Morocco, where there are few illegal firearms, although knife attacks and petty crime are more common.
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