Tymoshenko is serving a seven-year prison sentence after a conviction on charges she abused her authority when in 2009, as prime minister, she helped broker a natural gas deal with Russian energy company Gazprom.
Putin, who will be inaugurated as Russian president next week, was quoted by the Interfax news agency as saying the deal was checked "thoroughly" before it was signed.
"Let me say again that in our lawyers' opinion, neither the Russian nor the Ukrainian laws have been violated," he said.
Tymoshenko and many of her supporters believe the charges against her are politically motivated. She lost a bitter campaign to President Viktor Yanukovych in 2010.
She went on a hunger strike in late April after she claimed she was abused by prison authorities. Members of the European Union have said they wouldn't travel to Ukraine for an upcoming soccer tournament as an act of protest.
Putin said sports and politics shouldn't mix. He added that Tymoshenko could go to Russia for medical treatment if allowed to by Ukrainian authorities.