Emmanuel Abayisenga, a 42-year-old Rwandan, is also facing legal action for a separate incident in which he allegedly killed a priest in western France in 2021.
The court ruled that Abayisenga was not mentally sound at the time of the fire at the Cathedral of Saint Peter and Saint Paulwhen it handed down the sentence.
The court also banned Abayisenga from bearing weapons and staying in the western Loire-Atlantique region, where Nantes is located, for five years.
His lawyer, Meriem Abkoui, said her client’s answers in court occasionally “lacked coherence” and that his criminal responsibility was questionable.
She added that she was waiting for the results of psychiatric tests in the other legal proceedings against him, saying his trial for the priest murder could take place late next year.
Abayisenga, who arrived in France in 2012 and had been a volunteer for the local diocese, had admitted causing the blaze at the start of the hearing.
He said he had entered the cathedral to pray but then “lost control” after passing by a location in the building where he suffered a violent attack in 2018.
Speaking through an interpreter, he said he regretted what happened and asked for forgiveness.
Abayisenga has a history of unsuccessful asylum claims and received an order to leave France in 2019, which was said to have deeply troubled him.
The court acknowledged the defendant’s health issues, including hearing difficulties, incontinence, lung problems and eating disorders.
Prosecutor Veronique Wester-Ouisse said the defendant set fire to the cathedral knowingly due to “huge anger and a feeling of revenge linked to his administrative situation”.
Firefighters were able to contain the blaze quickly and save the main structure, but its famed 17th-century organ, which had survived the French revolution and bombardment during World War II, was destroyed.
Also lost were priceless artefacts, paintings and stained-glass windows that contained remnants of 16th-century glass.
The cathedral’s owners estimated the damage at more than €40 million.
The blaze in Nantes came 15 months after the devastating fire at the Notre-Dame cathedral in Paris, which raised questions about the security risks for other historic churches across France.