Lassana Bathily, 24, was in a stockroom when gunman Amedy Coulibaly burst in, killing four people.
At a ceremony attended by top French ministers, Mr Bathily said he was "proud and moved" to become a citizen.
The attacks in Paris killed 17 people, most of them at the offices of the Charlie Hebdo satirical magazine.
Mr Bathily's efforts came to light when he sneaked out and told his story after hiding people in a cold store room during the assault by Coulibaly.
"People say I'm a hero but I'm not a hero. I'm Lassana," he said at the ceremony.
"I'll stay the same. I would do the same again, because I was following my heart."
French Prime Minister Manuel Valls and Interior Minister Bernard Cazeneuve spoke at the citizenship event, thanking him for his bravery.
Mr Bathily has lived in France for the past nine years and applied for citizenship last year.
Meanwhile, Paris officials say they plan to sue US channel Fox News for a broadcast saying there were "no-go zones" in the French capital for non-Muslims.
Fox News has apologised for the reports but the Paris Mayor, Anne Hidalgo, told CNN her city had been "insulted".
In other developments, four men appeared in a Paris court suspected of providing Coulibaly with weapons and vehicles.
They are the first to face charges over the attacks.
Prosecutors have asked for the men to be detained longer and charged on terrorism charges. Their exact role in the attacks is unclear.
Earlier on Tuesday, five Chechens were arrested in southern France on suspicion of possessing explosives.
The men were not known to terrorism officers and a prosecutor warned not to "jump to conclusions".
Prime Minister Valls said the attacks highlighted deep divisions in French society, a "territorial, social and ethnic apartheid that has imposed itself on our country".
Three days of violence began in Paris on 7 January when brothers Cherif and Said Kouachi burst into the offices of Charlie Hebdo, killing 12 people.
Coulibaly killed four Jewish hostages on 9 January before being shot dead by police. He is also believed to have shot dead a policewoman the day before.
Almost 15,000 extra police and troops have been mobilised to boost security across France since the attacks.
Three days of terror
A Malian-born Muslim employee who hid customers from an Islamist gunman at a Paris kosher supermarket this month has been granted French nationality.