In his speech at a joint session of Parliament in Moscow, Putin stressed the historical and cultural ties between Russia and Crimea, said a CNN report.
Putin said Russia will not be deterred by Western criticism and also thanked China and India for their support. Crimea referendum was in full accordance with international law, said Putin.
"In our hearts we know Crimea has always been an inalienable part of Russia," he said. Crimea is an autonomous region within Ukraine with a majority Russian-speaking population. It has its own parliament, but the Ukrainian government had veto power over its actions.
The weekend referendum in Crimea put an end to the political instability and demonstrations in Ukraine for past several months, during which the President Viktor Yanukovych was overthrown.
A referendum which took place Sunday was alleged to have done in a haste. Almost 96.7% of the region's voters said they wanted to become part of Russia, according to the Crimean Electoral Commission.
Putin denied that Russia had been militarily involved in Crimea. "We have not used our armed forces in Crimea," Putin said. However, Ukraine government and international leaders have accused Russia of using military power in Crimea.
Russian did not enter Crimea in the current crisis, but "were already there" in accordance with previous international negotiations, stated Putin.
Crimea is a strategically important territory and must be kept strong -- something only Russia can do, he added.