WASHINGTON—President Trump named a China hard-liner to lead negotiations with Beijing, indicating the U.S. will pursue a tough stance in what is bound to be contentious talks over a trade dispute that has sent shivers through global markets.

Mr. Trump informed Chinese President Xi Jinping of his choice of Robert Lighthizer at their Saturday meeting in Buenos Aires, people familiar with the discussions said, pointing several times to the U.S. Trade Representative as the person who will face off with Beijing’s diplomats and using Mr. Lighthizer’s charts in presentations.

The remarks came as a surprise to a Chinese leadership that had maneuvered for months to deal with Treasury Secretary Steven Mnuchin, who had led initial rounds of talks, but failed to resolve the dispute over the past year.

The shift reflects the mixed messages the Trump administration is sending in the bitter contest between the world’s two largest economic powers. On the one hand, the administration is giving an expanded role to Mr. Lighthizer, who opposed China joining the World Trade Organization in 2001 and persuaded the president to ditch potential deals with Beijing over the past two years. He has been pressing for more tariffs on China as a way to build leverage over Beijing.

On the other hand, the president is touting that the U.S. made big strides toward settling the trade fight in his session with Mr. Xi—a message some advisers say is aimed as much at stock markets as at Beijing.