Police officer Lieutenant Colonel Arnaud Beltrame, who died saving the lives of hostages in a supermarket siege, will also be honoured in a separate national memorial in Paris in coming days.
Beltrame has been hailed as a hero after trading places with a captive.
This was the worst jihadist attack under Emmanuel Macron's presidency.
The gunman, 25-year-old Radouane Lakdim, had been on an extremist watchlist and was known to authorities as a petty criminal, but intelligence services had determined he did not pose a threat. He was shot dead by police.
The bishop of Carcassonne and Narbonne celebrated the Mass in the Church of Saint-Etienne in Trèbes, a rural town with a population of just 5,000.
Before the hostage-taking in Trèbes, Lakdim hijacked a car in nearby Carcassonne, shooting the driver and killing passenger Jean Mazières, a retired winemaker in his sixties from the town of Villedubert.
Lakdim drove to the Super U in Trèbes, where he killed the shop's chief butcher, Christian Medvès.
One-time local political candidate, Medvès, 50, was described as having the "joy of life".
Retired builder Herve Sosna, 65, happened to be at the butcher's counter when Lakdim mounted his assault and he too was killed.
Although police managed to free hostages from the supermarket, Lakdim held one woman back as a human shield. Beltrame volunteered to swap himself for her. He became the fourth victim when police raided the supermarket and he was mortally wounded.