Kirk said it was his job to work with Congress and others to ensure a level playing field for U.S. businesses and workers on a global stage.
"I believe investing in small businesses ... is a smart way to grow our economy because when small businesses grow, so do the families, the wages of those working families. Free and fair trade can be a way to make that happen," the former Dallas mayor said.
A huge challenge for his office is creating a trade agenda "that the public believes works for America's families, is transparent, and reflects our basic values," Kirk said.
The United States won't back away from the global marketplace and its leadership role in shaping trade policy, he said.
"We have an incredible opportunity before us. We also have a big challenge before us," Kirk said. "But I'm comforted by the fact that some of the finest talent and the best minds in the United States federal work force reside in the office of the United States trade representative."
Vice President Joe Biden administered the oath of office to Kirk.