The 20-year-old actress and the Baby hitmaker - who went public with their relationship in February 2011 - called time on their relationship last week.
A source told E! News: "They broke up about a week ago.
"Because of their crazy schedules, it was getting harder and harder to maintain a relationship."
The news comes after Justin, 19, was spotted on a date with 19-year-old Victoria's Secret model Barbara Palvin in New York earlier this week.
The pair - who met when Justin performed at the Victoria's Secret show in the city on Wednesday - enjoyed a trip to see the Lion King on Broadway.
The break-up is surprising as Justin recently revealed he won't go anywhere without Selena.
He said: "I never make [Selena] separate from me because I don't want her to ever feel like I'm ashamed of her. Because I feel like a lot of guys do that, especially in the business. They don't want to be seen with the girl, so they'll make them ride in separate cars. They'll do that 'get away' stuff. [Selena and I] get away together."