Police are investigating after damage was caused to a stained glass window at St. Andrew's Church, Narberth.
Nine small diamond-shaped panes of glass were found smashed by small stones.
Officers will now be stepping up patrols in the area in a bid to track down the vandals.
Youths have been seen congregating in the area and, as the incident comes only a few weeks after youngsters were warned about their behaviour in the grounds of the nearby castle, police and parishioners are concerned that they may have turned their attention to the church.
"We are appealing for neighbours to be vigilant and call the police as soon as they see any youngsters in the churchyard," said PC Dorian Evans, of the town's police station.
"We are also asking parents to make sure they know where their children are during the evening," he continued.
Anyone with any information is asked to contact the town's police station on 860389.