Hilton was arriving at court to testify against a man accused of trying to break into her home last August.
As Hilton and her boyfriend Cy Waits were walking into the courthouse just after 1 p.m., a man appeared and grabbed Waits by the neck. Hilton's bodyguard tackled him down immediately.
"Oh my God, that's another intruder that came to my house on a bike," Hilton said as the bodyguard held the man.
The man was identified as James Rainford, who had twice been arrested for showing up at Hilton's house and fighting her security guards - once in January and again in October.
Rainford was arrested, while Hilton was taken to a secure location inside the courthouse. When asked if he was trying to attack Hilton, he said no.
"I've already proposed to her, and her dad already said yes," Rainford said as he was being taken away.
After the incident, Hilton tweeted, ""That other psycho intruder just punched Cy in the back of the head as we were walking into the court house. So scary!"
In Hilton's break-in case, a jury of eight women and four men were selected earlier in the morning, and two women were selected as alternates on the jury.
Hilton was on the witness stand for about 20 minutes, testifying about the incident to the jury. Her boyfriend also testified.
Nathan Parada, 31, allegedly tried to break into the heiress' home last summer. She posted a Twitter photo when it happened.
Hilton says she and her boyfriend called police after seeing an intruder carrying two knives outside her Sherman Oaks home. When officers arrived, they reportedly found Parada banging on a door window.
Parada is charged with one felony count of attempted residential burglary.