That “something” is hard to identify definitively, but Mr. Greenblatt pointed to three likely factors: the increasingly divisive state of American politics, the emboldening of extremists, and the effects of social media. Some of the increase may also be attributable to better reporting of incidents.

The invigoration of the far right, including white supremacists and neo-Nazis, has been on display at events like a rally in Charlottesville, Va., in August that turned deadly when a man drove a car into a crowd of counterprotesters. A separate Anti-Defamation League report released last month found a more than 250 percent increase in white supremacist activity on college campuses in the current academic year. (The count released Tuesday does not include white supremacist incidents unless they had a specific anti-Semitic element.)

“The diminishment of civility in society creates an environment in which intolerance really can flourish,” Mr. Greenblatt said. And the platforms of social media, he added, have “allowed the kind of poison of prejudice to grow at a velocity and to expand in ways that really are unprecedented.”

The count by the A.D.L., an international organization that fights anti-Semitism and other forms of prejudice, includes three categories: harassment (1,015 incidents in 2017, up 41 percent from 2016), vandalism (952 incidents, up 86 percent) and assault (19 incidents, down 47 percent). The decrease in assaults was “the one piece of good news in this report,” Mr. Greenblatt said.