Us Weekly confirmed the 25-year-old singer and 22-year-old actress broke up "a while ago" after several months of dating.
"They split months ago," a source said. "Niall wrapped up his tour and had more free time, but Hailee's work schedule really ramped up, so they didn't have time for a relationship. They still have a lot of love for each other."
E! News said the One Direction singer and the Bumblebee actress have tried to keep their breakup quiet.
"Hailee and Niall were going strong over the summer but split a few months ago and have been trying to keep it low-key," an insider said.
"Hailee realized she had a lot on her place and her work schedule was insanely busy," the source said. "They really tried to make it work."
Horan and Steinfeld were first linked in January after they were spotted at the West End production of Hamilton. Horan had dedicated a sweet post to Steinfeld on her birthday the month previous.
"Happy birthday to the loveliest person on the planet and one of my best friends @haileesteinfeld," the star wrote.
Steinfeld's movie Bumblebee opens in theaters Dec. 21. The film is centered around the Transformers character of the same name.