The incident happened on the edge of the capital when police tried to stop two men on a motorcycle for a routine search.
“All of a sudden the motorcycle riders opened fire on them, one police head-constable has been martyred,” Muhammad Zia Ul Qamar, a spokesman for Islamabad police, told AFP.
Qamar said another policeman and a paramilitary soldier were wounded.
Police said they had launched a search operation to arrest the gunmen who fled the scene after the shooting.
Security in Islamabad, already on high alert, was tightened after a last month’s deadly assault on a university in Charsadda, in which 21 people were killed.
Pakistan launched a military offensive in 2014 that has reportedly killed thousands of militants and pushed the rest over the border to Afghanistan, resulting in improved security across the country.