Taking to Facebook, police said without Tiernan the 18-year-old may have been subjected to horrific abuse and awarded the young boy a Badge of Honour.
"Folks, this is Tiernan McCready. This is what a hero looks like," police said.
"Last week in the Bogside he saw three males grab an 18-year-old girl and try to get her in their van.
"Most ADULTS would be paralysed in shock, confusion or fear and fail to act till it was too late. Tiernan reacted instantly and shouted at the males and led the girl away to safety. He then told his mum who rang the police.
"Let that sink in.
"This could have been the start of a potentially serious and harrowing crime. That girl could have been my child, my sister, your child, your family member.
"The Bogside, Brandywell, and The Fountain Neighbourhood Team presented Tiernan with a PSNI Badge of Honour (and some Malteasers). But this doesn't go anywhere near to paying off the debt our community owes this boy. Our youngsters often get a bad rep but remember the vast majority are brave and big-hearted just like Tiernan."
Locals have taken to social media to praise and thank the young boy for his efforts, saying Derry is a safer place because of him
"Not all heroes wear capes. Such a brave and heroic boy. His family must be very proud. Because the whole of Derry is, and very thankful to him," one person wrote.
Another said: "Fantastic bravery young man! When many of your peers and elders wouldn't have been composed enough to do what you did. You should be very proud of yourself."
