"I made final decisions only after people's mood (in Crimea) has become clear," Xinhua quoted Putin told All-Russia Popular Front leaders.
The Kremlin, Putin said, had not been preparing to such developments in advance, and that he personally did not know for sure the mood of Crimea's population at the time.
Meanwhile, Putin admitted that Moscow had "held first sociological polls in secrecy" in Crimea, which showed 80 percent of the residents preferred to joining Russia. After the preparations for the referendum was announced, that number has further increased, he said.
"Those were surprising figures," Putin said, stressing that Russia would not act as assertively as it did unless the Kremlin knew for sure the aspirations of Crimean residents.
His remarks came after some domestic and foreign media reports said the Kremlin had long been planning for the annexation, with several schemes under table.
Putin signed a law on Crimea's accession to Russia March 18.
The president said he has been planning to meet with Crimea and Sevastopol leaders in near future and added that Crimea could become a money-making region in a few years.
An autonomous republic within Ukraine since 1992, Crimea held a landslide referendum on joining Russia in mid-March. Ukraine along with the majority of countries have rejected the results of the referendum.