Jean Zermatten, chairman of the U.N. committee on the rights of children, said Hind al-Barti, thought to be 15, was executed last week in Sanaa. He said it was "deplorable" that the execution took place despite assurances given in 2005 against such actions.
Zermatten's office said at least 15 juveniles were executed since 2006.
"We are not only outraged that child offenders continue to be executed in Yemen, in flagrant contravention of international law but we are also deeply concerned over the increased number of sentences of capital punishments pronounced against children," he said in a statement.
It wasn't clear for what offense the girl was executed.
Political unrest in the Middle East forced longtime President Ali Abdullah Saleh from power in early 2012. Jamal Benomar, a U.N. special adviser on Yemen, said an upcoming national dialogue conference could be the "crucible for a new Yemen."
Amnesty International last week, however, called on all parties to the conflict in Yemen to respect international law, stating that "as is so often the case, civilians were the main casualties."