South Yorkshire Police sent teenagers into a number of shops in the town centre as part of a 'test purchase operation' and said half of the stores sold blades to the underage customers.
A force spokesman said: "The Town Centre Neighbourhood Team conducted a knife test purchase operation. Sadly 50 per cent of shops failed and sold knives to persons under 18. "Appropriate action has been taken with those who sold knives but positive feedback was also given to those stores who passed and challenged the young people.
"Further work will be ongoing to keep Barnsley safe and support force-wide knife crime reduction."