The governor of Texas has ordered that face coverings must be worn in public across most of the state.
Gov Greg Abbott, who had pushed Texas' aggressive reopening of the state economy in May, previously said the government could not order individuals to wear masks.
His prior orders related to the virus had undercut efforts by local governments to enforce mask requirements.
But faced with dramatically rising numbers of both newly-confirmed cases of the Covid-19 virus and the number of patients so sick they needed to be hospitalised, Gov Abbott changed course with the statewide mask order.
The order requires "all Texans to wear a face covering over the nose and mouth in public spaces in counties with 20 or more positive Covid-19 cases, with few exceptions".
Violators can be fined up to $250 (£200).
There are exceptions for people who have a medical condition or disability, who are exercising outdoors, or who are participating in a religious service or voting.
Texas this week began its month-long early voting period for its primary elections.
Gov Abbott also gave mayors and county authorities the ability to ban outdoor gatherings of more than 10 people.
In his order and a statement, Gov Abbott said wearing a mask is a proven method to slowing down the virus' spread and said if Texans comply "more extreme measures may be avoided".
Gov Abbott has said he does not want to roll back his previous orders to reopen the economy. But last week he moved to reclose bars and limit dine-in capacity in restaurants to 50%.
"We have the ability to keep businesses open and move our economy forward so that Texans can continue to earn a paycheck, but it requires each of us to do our part to protect one another - and that means wearing a face covering in public spaces," he said.
Gov Abbott has been under extreme pressure from both Democrats and Republicans on his reopening plans.
Democrats, most notably the leadership of the state's largest cities, have complained Gov Abbott reopened Texas too quickly and have pointed to record numbers of confirmed new cases and hospitalisations.
Conservative politicians condemned his early moves to combat the virus, including business and school closures and a stay-at-home order that expired in May.
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"The move to mandate face masks comes far too little, far too late for Governor Abbott," Texas Democratic Party communications director Abhi Rahman said.
"Texans are still getting sick. Families are still suffering ... All of this could have been prevented if Governor Abbott had listened to experts and medical professionals in the first place."
