Pakistan, a nuclear-armed nation of 190 million people, is plagued by criminal gangs, sectarian militants and a Taliban insurgency.
Threats and attacks on civilian judges and lawyers prompted the government to set up secret military courts in February to try militant suspects, following a Taliban massacre of 134 children at an army-run school last December.
"Our special task force has targeted additional session judge Tahir Khan Niazi in Rawalpindi successfully," Taliban spokesman Muhammad Khorasani said in an emailed statement. He did not say why Niazi was targeted.
Abdul Hafiz, a clerk at the sessions court in the city next to the capital, Islamabad, confirmed Wednesday's killing.
"Some people came to his home and fired on him," he said, Reuters reported.
The government says militants usually walk free because civilians are too scared to convict them. Lawyers and judges agree security is a problem, but they also complain of outdated laws, poorly trained and poorly paid police, and political interference.
Saroop Ijaz, the Pakistani representative of rights group Human Rights Watch, said the government must reform the long-neglected criminal justice system. But so far, there has been little sign of progress, he said.
"The government needs to have a public conversation on what is being done to fix that broken system," he added.