One way or another, Donald Trump’s election could change Brexit.

For those already anxious about Britain’s departure from the European Union, the election sharply increases uncertainty as the U.K. casts itself adrift from the bloc. Others expect Mr. Trump will prioritize relations with the U.K. and ease the pathway out.

The big economic unknown is over trade. The power of the presidency to set trade policy is extensive and Mr. Trump has said America’s current trade arrangements swell its trade deficit and hurt American jobs and wages.

His election was the final nail in the coffin of the Trans-Pacific Partnership, a trade deal with Asia. He has said he would renegotiate or withdraw from the North American Free-Trade Agreement with Mexico and Canada. And a trade-and-investment deal being negotiated with the EU is now in the freezer, says EU Trade Commissioner Cecilia Malmström.

Mr. Trump has also threatened actions, including possible big tariff increases, to combat what he calls unfair trade from China.