Police say it was lucky that it did not lead to a collision.
Two youths were seen on the bridge at Ruabon at the time the rock hit the windscreen around 8.30am yesterday .
Police say that the lorry was travelling southbound on the dual carriageway.
A North Wales Police spokesman said: "We are appealing for information after a lorry travelling south on the A483 at Ruabon was hit by a rock thrown from a bridge by two youths at around 8.30am today.
"The windscreen was cracked, but fortunately there were no injuries or collision. We need to trace these youths. Call 101 ref X153625."
Alternatively, contact Crimestoppers anonymously on 0800 555111 or at crimestoppers-uk.org