Putin's confession immediately led Georgia to accuse the Kremlin of starting the brief but bloody war on 2008.
Putin's comments appeared to be a bid to shield his Kremlin predecessor Dmitry Medvedev from criticism following an online documentary that accused the former commander-in-chief of procrastinating over the decision to use force against Georgia.
Speaking on the fourth anniversary of the war, Putin said he had approved the contingency plan to counter a possible attack from Georgia months before the conflict broke out.
"There was a plan, it's no secret in my opinion," Putin said at the Kremlin in televised remarks.
"It's within the framework of this plan that the Russian side acted. It was prepared by the General Staff at the end of 2006 or the beginning of 2007."
"It was approved by me, agreed with me," said Putin.
"Moreover, within the framework of this plan the training was conducted of South Ossetian militiamen," he said, adding that the men proved very helpful during the conflict.
Putin, who at the time served as prime minister after the maximum two consecutive Kremlin terms allowed by the Constitution, said he called Medvedev twice to discuss the conflict.
Georgia said Putin's revealing comments proved that the Kremlin was responsible for the conflict.
"Such a confession never happened before and it's a paradox that even today some people are arguing about who was the aggressor," the head of Georgia's National Security Council, Giga Bokeria, told journalists.
The two neighbours clashed when President Mikheil Saakashvili's military attempt to reassert control over Moscow-backed South Ossetia was crushed by Russian troops who pushed deep into Georgian territory.
After the war that stunned the West, Russia recognised South Ossetia and the fellow Georgian rebel region of Abkhazia as independent, a move that has been followed by only a handful of other far-flung states.
In a documentary making the rounds on the Russian Internet, retired army chiefs accused Medvedev of dithering at the start of the war, claiming Russia could have avoided multiple casualties if not for his indecision.
"The initial command came with great delay," Yury Baluyevsky, chief of General Staff between 2004 and June 2008, said in the documentary.
"To put it mildly, everyone was afraid of something" before they got a "kick in the butt" from Putin, Baluyevsky said.
Medvedev, speaking from South Ossetia where he arrived earlier Wednesday for commemoration ceremonies, appeared to defend himself, saying he had to make a series of "difficult decisions."
"It was done in a timely manner, it was possible to prevent much higher casualties rather quickly," he said in the capital Tskhinvali where he was greeted with applause from locals. "These decisions were correct and just."
Speaking on Tuesday, Putin told a reporter he did not see the documentary about the war but said a decision to use force should never be taken lightly.
"Before making such decisions you have to think 10 times," he said, adding that Russian authorities took three days before deciding to repel Georgian troops.
Saakashvili marked the anniversary by laying a wreath at a military cemetery in Tbilisi and meeting the families of some of the soldiers killed.
National flags have been lowered across the country for two days and campaigning for October's parliamentary polls has been temporarily suspended.
The fighting is estimated to have killed at least 250 people and forced some 118,000 others from their homes
