The storm has dumped record rainfall over the past four days, leaving large parts of the city underwater and at least 15 people reported dead.
Houston's mayor, Sylvester Turner, said the curfew was essential to head off looting in the US fourth biggest city.
President Donald Trump visited Texas on Tuesday to survey the damage brought by Harvey, calling the storm "epic".
The curfew is in place between 00:00 and 05:00 local time (05:00 and 10:00 GMT). Relief volunteers, first responders, and those going to and from work are exempt.
Mr Turner said the curfew would help prevent "property crimes" against evacuated homes, and was in force "only to prevent potential criminal acts".
City officials have reported instances of looting, armed robberies and of people impersonating police officers.
President Trump earlier arrived in Corpus Christi, where Hurricane Harvey first made landfall on Friday, with First Lady Melania Trump,
He will not be visiting Houston. The White House said he did not want his visit to disrupt the emergency response.
Mr Trump has already declared a federal state of emergency in both Texas and nearby Louisiana, where Harvey is expected to make landfall again on Wednesday morning.
The move releases emergency government funding and other assistance.