The warning comes days after workmen narrowly dodged a blade thrown from the window of a tower block in Clydebank, West Dunbartonshire.
Construction workers were in a forklift truck at West Court, Littleholm Place, when they saw the knife hit the ground, just before 1pm on May 12.
The workers saw a man hanging out of a window in the moments after the knife was thrown.
A letter sent to tenants in that block, and two others, from West Dunbartonshire Council, said: "Very recently, we have become aware of incidents where knives have been thrown at persons below.
"This is unacceptable and is now in the hands of the police and our ASB (anti-social behaviour) team.
"The perpetrators will be dealt with through the courts."
Martin Docherty-Hughes, MP for West Dunbartonshire, said: "This sort of idiotic behaviour is completely unacceptable.
"Dropping items from high-rises is dangerous and irresponsible and could result in serious injury.
"Thankfully incidents like this are extremely rare, and it's a relief that nobody was hurt on this occasion.
"I'd hope that any residents who witness such reckless behaviour do the right thing and report it."