Police and relatives say Nahed Hattar was gunned down shortly before arriving for a court hearing in Amman. They said the killer was arrested at the scene.
Mr Hattar was arrested in August after sharing a cartoon on social media which allegedly depicts God in heaven being treated as a servant by an Arab man, who is lounging in bed with two women and asking for wine.
Relatives said the cartoon was meant to illustrate what Mr Hattar viewed as the immoral religious views of Islamic State extremists.
His post was quickly deleted after many angry responses.
Jordan is a majority Muslim country, where only 2% of the population are Christians.
