WASHINGTON—The U.S. on Friday restricted the ability of Venezuela to tap American debt markets for funding, marking a significant escalation in Washington’s economic crackdown against Venezuelan President Nicolás Maduro’s cash-strapped government.

President Donald Trump’s executive order prohibits institutions in the U.S. from trading new bonds with Mr. Maduro’s government, including through the state-owned oil company, Petróleos de Venezuela SA. It doesn’t curb trading of existing Venezuelan bonds in secondary markets and...