At 01:00 local time, unidentified armed men attacked the president’s residence in Port-au-Prince, according to Claude Joseph (05:00 GMT).
Martine Mose, the First Lady, was hurt in the attack.
“All efforts have been made to ensure the state’s continuation,” Mr Joseph added.
He has now seized charge of the poor Caribbean country and asked everyone to keep calm.
He termed the president’s assassination a “heinous, inhumane, and cruel crime.”
Boris Johnson, the UK Prime Minister, said that he was “shocked and saddened” by Mr Mose’s murder, calling it “an terrible crime” and urging calm. The assassination was described by the White House as a “horrific crime.”
The Dominican Republic’s border with Haiti has been ordered “immediately closed.”
Since February 2017, Jovenel Mose, 53, had been in power. He faced charges of corruption and was confronted by waves of frequently violent anti-government rallies during his tenure in power.
People sought his resignation in large rallies in the capital and other towns early this year.
Mr. Mose’s five-year tenure should have expired on February 7, 2021, five years to the day after his predecessor, Michel Martelly, stepped down, according to Haiti’s opposition.
However, elections were postponed for a year following Mr Martelly’s resignation, and Mr Mose argued that he had one more year to serve because he did not enter office until February 2017.
Mr Mose claimed an effort to assassinate him and topple the government was stopped in February of this year, on the day the opposition wanted him to leave office.
Haiti is one of the poorest countries in the Americas due to chronic instability, dictatorships, and natural catastrophes.