Colin Farrell got an unexpected, and unwanted, visitor at his home in Los Angeles this weekend -- and now the guy's been arrested and placed on a psychiatric hold.
Law enforcement sources tell TMZ ... a man showed up to Colin's home Saturday afternoon hoping to catch a glimpse of the actor ... but things got weird when he walked right up to the front door.
We're told Colin was home at the time and got freaked out ... so he called his assistant who called cops.
The fan was quickly taken into custody and after cops spoke with him -- they decided to put the suspect on a 5150 psychiatric hold.
Police haven't released the suspect's name -- but our sources say he is not a U.S. citizen, and one of the reasons he came to L.A. was specifically to see Colin.