The 18-year-old was walking along Lecky Road when a van pulled up alongside and men inside tried to pull her inside.
But quick-thinking Tiernan, who spotted the incident, shouted out at the gang, who quickly fled the scene and the teenager was able to escape their clutches.
PSNI Foyle say on its Facebook Page: “Folks, this is Tiernan McCready. This is what a hero looks like.
“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 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.”