All of Maduro's assets subject to US jurisdiction have been frozen and Americans are barred from doing business with him, the US Treasury Department's Office of Foreign Assets Control said on Monday.
The imposition of sanctions on a head of state is an infrequent measure and can be symbolically powerful; other countries may decide to take similar action.
President Donald Trump's national security adviser H.R. McMaster said on Monday that the election to form a constituent assembly to rewrite the constitution was an "outrageous seizure of absolute power."
"By sanctioning Maduro, the US makes clear our opposition to the policies of his regime and our support for the people of Venezuela who seek to return their country to a full and prosperous democracy," Treasury Secretary Steven Mnuchin said in a statement.
"Maduro is a dictator who disregards the will of the Venezuelan people," Mnuchin said.
"Anyone who participates in this illegitimate (constituent assembly) could be exposed to future US sanctions for their role in undermining democratic processes and institutions in Venezuela," Mnuchin said.
Maduro riposted later on Monday, saying he was "proud" of having had sanctions imposed on him by the US.
"The sanctions are for having called on the people to vote freely to elect a Constituent Assembly," Maduro said at the National Electoral Council.
"I'm proud of these sanctions, Mr Emperor Donald Trump [...] Trump is making the biggest mistake of his life by interfering with Venezuela," he said.
It is unclear if Maduro has any assets in the US.
The US and others have said they do not recognize the results of Sunday's election, which appointed a new "Constituent Assembly" that supersedes the the opposition-controlled National Assembly.
Maduro celebrated the election of the new legislative superbody but further protests have been called by the opposition who hold a majority in Congress.
Ten people died in violence surrounding Sunday's poll.
Last week the US imposed sanctions on several Venezuelan officials, including the chief judge and seven other members of Venezuela's pro-Maduro Supreme Court after they ruled to annul Congress earlier this year.
jbh/jm (Reuters, AFP, AP, dpa)