Alexandria Ocasio-Cortez, the self-described Puerto Rican “girl from the Bronx” who is set to become the youngest congresswoman in American history next month, added another layer to a complex personal story: She has Jewish heritage.

Ms. Ocasio-Cortez, a New York Democrat, made the reveal on Sunday evening at a Jackson Heights synagogue in Queens, telling the crowd that “a very, very long time ago, generations and generations ago, my family consisted of Sephardic Jews.”

She dated her ancestry to those who had to flee Europe during the Spanish Inquisition more than 500 years ago, and sought refuge in the New World. “Some of those people landed in Puerto Rico,” she explained.

“As is the story of Puerto Rico, we are a people that are an amalgamation,” she said. “We are no one thing. We are black; we are indigenous; we are Spanish; we are European.”