Stella, who won The Apprentice in 2010, is said to have called in the police after receiving frightening phone calls from a mystery man.
According to The Mirror, the mum-of-two has been left fearing for her safety after the culprit threatened to "go round to her house".
A friend told the newspaper: "She is very emotional and is seriously concerned for her safety."
Reveal contacted Stella's rep but they had no comment to make. Police confirmed to The Mirror that they are investigating.
Stella hit the headlines last year when she took Lord Alan Sugar to an employment tribunal, claiming she was forced to resign from the £100,000 job she had won on the show because it was "not a role of substance".
The 34-year-old businesswoman, based in Kent, lost the case.
Towards the end of last year, Stella began dating former kids' TV presenter Toby Anstis. Their relationship fizzled out after two months.