Port-au-Prince had been suffering an increase in violence as gangs battle one another and police for control of the streets.
The bloodshed is driven by worsening poverty and political instability. President Moïse faced fierce protests after taking office as president in 2017, with the opposition accusing him this year of seeking to install a dictatorship by overstaying his mandate and becoming more authoritarian – charges he denied.
“All measures are being taken to guarantee the continuity of the state and to protect the nation,” Mr Joseph said.
President Moïse had ruled by decree for more than a year after the country failed to hold legislative elections and wanted to push through a controversial constitutional reform.
The US Embassy said in a statement it would be closed on Wednesday due to the “ongoing security situation”.
The United States had on 30 June condemned what it described as a systematic violation of human rights, fundamental freedoms and attacks on the press in Haiti, urging the government to counter a proliferation of gangs and violence.
Dominican Republic President Luis Abinader held an emergency meeting early on Wednesday about the situation but had yet to issue a statement.