A boy aged just 14 dumped a 15-inch “Rambo” knife as he tried to run away from police in Chelmsley Wood.
The teenager was among a group of youths who fled during a police crackdown on knife crime and youth violence, Operation Guardian.
It happened in Chelmsley Wood on Saturday afternoon, January 16.
The group ran when they approached by police over yob behaviour.
Two of them stopped and spoke to police.
The 14-year-old was detained and was found to have thrown away the knife as he ran.
West Midlands Police tweeted details of the arrest.
Officers posted: “PC Malhi, PC Gordon & PCSO Monger on #OpGuardian patrols in #ChelmsleyWood come across a group of youths causing ASB.
“As we approach some run off. Two stop and comply, one 14yr old male who ran, discarded a 15 inch Rambo knife before being detained!”