Police arrested the hostage-taker after hours-long standoff.
"All the hostages in Kharkiv have been released and the attacker arrested. I have instructed the local authorities to provide all necessary assistance to those affected," the Russian news agency TASS reported, citing Ukraine's President Pyotr Poroshenko, as saying, in a Twitter post.
None of the hostages were harmed.
Five of the hostages which included three women and two children were released early.
According to Ukrposhta Deputy CEO Oleksandr Pertsovskyi, three of 11 captives were the workers of Ukrposhta. The others were members of the public, the local media reported.
In earlier reports, the police said that the man stated that he had firearms and explosives, but made no immediate demands.