The 61-year-old actor was jogging in Central Park, New York, when he heard a woman shout that the group of youngsters were throwing stones at a pup.
True to form, Liam "charged down the path" towards the boys and shouted at them to leave the dog alone.
The incident then came to an end when the boys dropped their stones and ran – seemingly helped along by the appearance of a police officer.
Of course Liam didn't leave it there – we've seen him scour the streets of Paris in search of his daughter in hit movie Taken – the star went to comfort the frightened puppy.
According to the source, "Liam just shook his head and approached the shivering dog, stroking it while the policeman assessed its injuries."
This isn't the first time Liam has shown his softer side. The Non Stop star has recently been active in his support of New York's horse-drawn carriage industry, which the mayor wants to shut down.
On a more personal level, last month he opened up about his ongoing grief five years after the tragic death of his wife Natasha Richardson. The actress passed away following a skiing accident, and her devastated husband is still coming to terms with his loss.
"(Her death) was never real," Liam said in a poignant interview with CBS' Anderson Cooper for 60 Minutes. "It still kind of isn't.
"There's periods now in our New York residence when I hear the door opening, especially the first couple of years… anytime I hear that door opening, I still think I'm going to hear her."