The U.S. Department of Energy is debuting a supercomputer from International Business Machines Corp. Friday that it says is the world’s fastest and leapfrogs a Chinese machine that has held the top spot for the last two years.

The competition for the world’s fastest computer, ranked twice a year by computer scientists in the U.S. and Germany, is a point of national pride. The last time a U.S. computer held the top spot was in November 2012.

The...