New York Mayor Bill de Blasio met U.S. Sen. Bernie Sanders in Brooklyn last month as he continued to weigh an endorsement in the 2016 presidential contest and planned to host a candidates’ forum in Iowa later this year.

Mr. de Blasio, who has praised the liberal Vermont senator for his economic positions, met with Mr. Sanders as he traveled to New York for a television appearance. The mayor’s camp didn’t list the meeting on his public schedule.

“We...