Mr Moise's widow, first lady Martine Moise, is in hospital, interim premier Claude Joseph said.
Mr Joseph condemned what he called a "hateful, inhumane and barbaric act", adding that Haiti's National Police and other authorities had the situation in the Caribbean country under control.
The killing late on Tuesday comes amid deepening political and economic instability and a spike in gang violence.
The nation of more than 11 million people had grown increasingly unstable and disgruntled under Mr Moise's rule.
Its economic, political and social woes have deepened, with gang violence spiking heavily in the capital of Port-au-Prince, inflation spiralling and food and fuel becoming scarcer at times in a country where 60% of the population makes less than two US dollars a day.
These troubles come as Haiti still tries to recover from the devastating 2010 earthquake and Hurricane Matthew that struck in 2016.
Mr Moise, 53, had been ruling by decree for more than two years after the country failed to hold elections, which led to Parliament being dissolved.
Opposition leaders have accused him of seeking to increase his power, including approving a decree that limited the powers of a court that audits government contracts and another that created an intelligence agency that answers only to the president.
In recent months, opposition leaders demanded the he step down, arguing that his term legally ended in February 2021.
Haiti was scheduled to hold general elections later this year.
Reporting by PA