Constable Ghulam Hassan was shot in the neck by terrorists from close range at Anantnag bus stand, a police official said.
The injured policeman was taken to district hospital in Anantnag from where he was referred to 92 Base Hospital of the Army in view of his "critical" condition.
Constable Hassan was unarmed at the time of the attack, the official said.
Terrorists have been targeting security forces in hit and run attacks in the valley especially in crowded places. In most of these attacks, terrorists have used pistols that are easy to hide and carry.
Earlier in the day, two terrorists were killed in an encounter with security forces in the Pulwama district of Jammu and Kashmir. The gun battle started early this morning after security launched an operation in Bahmnoo area of Pulwama, 40 km from capital Srinagar, when they received information about the presence of three terrorists.
While two of them have been killed, one is still firing at the security forces.
With inputs from PTI and IANS