The socialite singer and Niarchos have been an item for many years but they started getting serious about their relationship last year.
Niarchos joined his girlfriend for her birthday party in Las Vegas last month and Hilton left no doubt she was dating the Greek-born stud.
But now friends are telling Life & Style magazine the romance is over because 26-year-old Hilton thinks her 21-year-old boy toy is "too insecure and immature."
The couple reportedly broke up for good at the weekend.
A loose-lipped guest tells the publication, "They kissed on the sofa like teenagers. They were so cute, and the chemistry between them was electric."
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