Sources with direct knowledge tell TMZ ... the actor -- famous from "Kingdom," "Hostage," "The Ruins," and other shows/films -- called 911 Sunday evening to report a suspicious man who was going around banging on doors and talking to himself in his L.A. neighborhood.

We're told after Jon got off the phone with cops, he went outside after losing sight of the guy -- and when he got out there ... he saw that one of his neighbors' front doors was wide open, so he raced over to make sure everything was okay.

Our sources say Jonathan went inside, found a little girl in one part of the house -- whom he swooped up, only to come across her mother and another child in yet another part of the home. We're told the mom was completely panicked and frantic ... not knowing what was going on.

We're told the family was not even aware that their door was open and that the man entered the home -- something cops discovered to be the case later on. Obviously, it was a scary situation, and Jon calmed them down ... quickly explaining what was happening.

Soon enough, he led them to safety ... walking out of the house with the girl in his arms, and mom and other kid close behind. He got them far away from the house, and cops showed up within minutes, including a police helicopter. They went in and found the dude, placing him in custody. Law enforcement sources tell us the man was placed on a 5150 hold.

In terms of why he decided to spring into action ... Jonathan's camp tells us he simply wanted to do what he hopes someone would do for him and his family in the exact same situation. We're also told Jonathan considers the cops to be the real heroes here.