Grande Paroisse, a Total subsidiary specialising in chemicals, was found guilty of negligence and serious misconduct over the disaster, which struck on 21 September, 2001, 10 days after the September 11 attacks in the US.
The plant was completely levelled by the blast, and the company was fined €225,000.
The plant's former manager Serge Biechlin was also convicted over the tragedy, which left 8,000 injured and caused widespread damage.
Biechlin, 72, was given a 15-month suspended sentence and a fine of €10,000 for manslaughter.
Both defendants announced plans to appeal the outcome of the third case in 16 years over the explosion, which scattered debris over several kilometres, injuring people far from the site.
The prosecution and lawyers for 2,700 civil plaintiffs argued that the blast was an accident linked to poor waste management at the factory.
Several reports suggested that workers at the site made a mistake when mixing chemicals.
The defendants had mooted other possible causes such as a terror attack or the presence of chemical deposits in the ground.
Biechlin and Grande Paroisse, which is now known as GPN, were cleared of responsibility at a first trial in 2009.
That finding was overturned on appeal in 2012 but the appeals court's guilty verdict was in turn struck down over concerns about the impartiality of one of the magistrates, leading to a re-trial.
Total says it has paid out €2.5 billion to the victims in compensation but has repeatedly denied any criminal responsibility.