"The state, using an absolutely legal market mechanism, is protecting its interests," Putin told a press conference in the Kremlin.
"I consider this perfectly normal. Some market participants got multi-billion state assets using different tricks [in the years following the collapse of the Soviet Union], including some violations of then-existing legislation".
In a secret, no-bid sale, Rosneft purchased an obscure company that a couple of days earlier had bought.
The deal means the Kremlin has taken over Russia's largest oil company following a relentless 18-month official crackdown. This is the first major renationalisation of the post-Soviet era.
Putin's deputy chief of staff and longtime confidant, Igor Sechin, is Rosneft's chairman.
On Friday the virtually unknown company, Baikal Finans Group, had been registered to bid in the auction. On Sunday it made the winning bid of $US9 billion ($A11.8 billion) for Yuganskneftegaz.
Yukos an independent analysts have said Yuganskneftegaz is worth much more.
