The 62-year-old, who has dual nationality, was sentenced to three years in jail for attempted subversion in 2011 but was released after 17 months and ordered to serve three years' house arrest.
Since his release he has continued to publish articles he describes as "peaceful" but critical of the government on social media.
But he has received a copy of a stamped letter from President Tran Dai Quang confirming the "removal of Vietnamese citizenship".
According to the letter, the decision was based on article 88 of the criminal code, which criminalises propaganda against the state, and article 91, which outlaws moving abroad with a view to oppose the government.
Revoking his citizenship effectively renders his status in Vietnam illegal.
"I am very upset and I'm waiting, I'm waiting to be expelled," he told the AFP news agency Sunday.
Hoang went to France in 1973 but returned after 27 years to settle in Vietnam, where he worked as a mathematics lecturer at the Polytechnic University of Ho Chi Minh City.
He is the only dissident to have his citizenship revoked in modern memory.
Hoang has vowed to appeal against the decision and has sent a letter to the French embassy in Vietnam expressing his wish to revoke his French nationality, which he hopes could force the Vietnamese government to reverse its decision.
He says he must stay in Vietnam to care for his disabled older brother and his wife's elderly mother.
Vietnam routinely jails bloggers, lawyers and activists accused of anti-government activity.
All media are state-owned but many dissidents have moved onto social media platforms in recent years to air criticism.