The victim was shot in the chest and stomach with a pump-action shotgun and was in a critical condition, police and the newspaper said.
The shooter fled the scene and was still at large several hours after the attack, which occurred around 10:15am (1445 IST).
"I was just arriving for work and I saw a man lying on the ground, holding his stomach and with blood everywhere," Liberation journalist Anastasia Vecrin told AFP.
Police were quickly deployed at all media offices in Paris for fear of further attacks.
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Police are investigating a possible link between the two incidents.
Liberation executive Nicolas Demorand said the attack had left staff horrified.
"When you have someone with a shotgun coming into a newspaper's offices in a democracy, it is very, very serious, whatever the mental state of the person," Demorand told AFP.
"If papers and other media have to become bunkers, something has gone wrong in our society."
The newspaper said the injured man worked as an assistant to one of the photographers for Next, a supplement of the newspaper.