Mila Kunis Alleged Stalker TERRORIZED Her

Mila Kunis' alleged stalker has been unrelenting in his pursuit of the actress ... going as far as breaking into her house, TMZ has learned.

Stuart Dunn was detained Friday, for allegedly stalking the 28-year-old actress in front of her gym ... and Friday was the third day in a row he showed up at the facility looking for her.

Law enforcement sources tell TMZ ... Dunn was arrested for allegedly burglarizing Kunis' home in January. He pled no contest to unauthorized entry into a home and given 36 months probation. Dunn was also ordered to stay clear of Kunis for 3 years.

It appears Dunn has violated the stay away order and he could be jailed as a result. We're told right now Dunn is in a mental facility for evaluation.