Gottheimer (D-5th Dist.) took the oath on the House floor with the rest of the chamber, and then had his family join him at a private reenactment with House Speaker Paul Ryan (R-Wis.).
"It's really humbling," Gottheimer said in a brief interview. "You take the oath and you realize, 'What a enormous responsibility.' We have a lot of work to do."
In between the two swearing-in ceremonies, he hosted more than 200 supporters at his new congressional office. New Jersey's two U.S. senators, Cory Booker and Robert Menendez, were among those stopping by.
Gottheimer said his top priorities were rebuilding U.S. public works and overhauling the tax code.
He said he would be meeting with mayors and other local government officials to find out their needs and then work to get federal grants for them, an effort to reduce New Jersey's balance of payments deficit. New Jersey taxpayers get just 48 cents back for every $1 they send to Washington every year, less than all but three states, according to the consumer site WalletHub.
"How can we actually get back what we're paying in taxes?" Gottheimer said.
Gottheimer won his seat by defeating seven-term Republican incumbent Scott Garrett in a nasty race after raising $4.8 million, more than any other House candidate from New Jersey ever had. The only congressional candidate from the state with more cash to spend was Tom MacArthur, who lent his campaign $5 million to win an open seat in 2014.