The 40-year-old man, who was shot and wounded by police after lunging at the officer, had shouted that he was "a soldier of the caliphate" during the attack on Tuesday and that he was acting in revenge "for Syria".
The video pledging allegiance to the jihadist group was found in a police search of an apartment he was renting in Cergy outside Paris, the source in the investigation said.
The man was placed in custody in hospital, where he is being treated for a gunshot wound to the chest.
Documents found on the attacker identified him as an Algerian doctorate student studying the media at a university in eastern of France.
Government spokesman Christophe Castaner told RTL radio on Wednesday that the attacker had "never showed any sign of radicalisation".
The 22-year-old police officer sustained minor neck injuries in the assault in the packed square at one of France's most visited tourist attractions.
The attack comes as France is on high alert after jihadists killed seven people in London on Saturday.
France is still under the state of emergency imposed after the November 2015 attacks in Paris, when ISIS jihadists killed 130 people in a night of carnage at venues across the city.