Kat Von D had a frightening night Saturday -- and frightening is a gross understatement -- an intruder was wandering around her house as she and her son slept.
Law enforcement sources tell TMZ ... Kat was sound asleep at around 10:15 PM when something awakened her ... she saw a beam of light moving about the house.
Kat took her 3-year-old, dashed out and 911 was called. Police came with sirens blaring and searched the house ... and what they found was shocking.
A man in his 30's was upstairs, just walking around. It's unclear if he knows it was Kat's house or what his intentions were. He told cops he was buying the house and needed to use the bathroom.
Police took the guy into custody ... he was booked for residential burglary.
As for how he got in ... he jumped a fence. It's unclear if he came in through an open door or window, but there were no signs of forced entry.
What's interesting ... Kat recently put her $15 million home on the market, so the story he was buying the house may have been something he saw ... which suggests he knew who owned the house.