A senior police officer blamed the separatist Tamil Tigers for the assassination in the Sri Lankan capital, Colombo, the BBC reported.
The Tamil Tigers have been observing a ceasefire since 2002.
A spokesman for Kumaratunga said the state of emergency was intended to facilitate "the free deployment of emergency troops."
Kadirgamar, 73, was shot near his heavily-guarded home on Friday evening, said the BBC. Police believe one or two snipers were involved, and two men were reported arrested in connection with the attack.
Kadirgamar was rushed to a hospital, where he died despite undergoing emergency surgery.