The man, who was arrested on Monday, is the second to be held in connection with the murder of 85-year-old Jacques Hamel, whose throat was slit in front of worshippers while he was celebrating mass in the town of Saint-Etienne-du-Rouvray on July 26.
A police source said the arrested individual had been in contact with the cleric's killers, Abdel Malik Petitjean and Adel Kermiche, who had pledged allegiance to the Islamic State armed group.
The perpetrators, both aged 19, were shot dead by police after a hostage drama in which a worshipper was seriously wounded.
The attack – the first committed in the name of IS armed group against a church in the West – came less than two weeks after a Tunisian drove a 19-tonne truck into a crowd celebrating Bastille Day in Nice, killing 85 people and wounding more than 300.
A cousin of Petitjean named as Farid K. was remanded in custody on July 31 on charges of “criminal association in connection with terrorism”.
The public prosecutor said the 30-year-old “was fully aware of his cousin's imminent violent action, even if he did not know the precise place or day.”
Hamel's killing, which follows a string of attacks in France over the past 18 months, has raised questions about security failures.
Kermiche, who lived near the church, wore an electronic tag and was allowed to leave home on weekday mornings under his house arrest terms.
Investigators have questioned several people over the killers' links with groups recruiting for jihad in Syria.
A 20-year-old man has been charged with trying to travel to Syria with Petitjean in June.
- with AFP