RWE said as much as 353 billion cubic feet of natural gas per year would flow from Poland to Ukraine under the terms of a contract signed in 2012.
"RWE, via its trading and gas midstream arm RWE Supply & Trading, is the first European supplier to commence gas deliveries to the Ukraine in 2014," the company said in a statement. "Today RWE re-starts deliveries to the state-owned company Naftogaz of Ukraine with gas from its pan-European portfolio."
The German energy company said additional supplies to Ukraine could come from Slovakia in the coming months.
Russian President Vladimir Putin warned that European energy security was at risk because of mounting gas debt obligations in Ukraine.
Russian energy company Gazprom in 2009 cut gas supplies to Ukraine because of contractual disputes. That left European consumers in the cold for several weeks because, while Russia meets about a quarter of Europe's gas needs, most of those supplies run through the Soviet-era pipeline network in Ukraine.
The deal that ended the impasse left Ukraine with some of the highest gas bills in the region. RWE said "deliveries to the Ukraine are based on European wholesale price levels including delivery costs to the Ukraine."