Sehwag reached the feat in style, hitting off-spinner Saqlain Mushtaq for a six over mid-wicket after the lunch break on the second day of the first Test against Pakistan in Multan.
Resuming the day at 228, Sehwag took his score to 292 at lunch. He had to wait for some time in the post lunch session to achieve the distinction. His knock came in 364 balls and included 36 fours and six sixes.
Immediately after reaching the milestone, he waved the bat to the crowd and was greeted by batting partner Sachin Tendulkar.
The Delhi player joined 16 others in the elite triple-century makers club.
Earlier, when on 282, he overtook V V S Laxman's 281 made against Australia at Kolkata in 2001, to be the highest run scorer for India.
With a record triple century under his belt, Virender Sehwag said his delight at achieving the feat was tinted with disappointment as he could have attempted to overtake Matthew Hayden's world record.
"I am thrilled. I can't believe that I have done it. May be it will sink in after a couple of days. All I can say is that it is a great feeling. It is very special because it has come against Pakistan," was Sehwag's reaction at scoring 309, the only triple ton by an Indian ever in Test cricket.
But the flamboyant opener admitted that he had Hayden's world record of highest individual Test score of 380 in his mind and was disappointed to have missed it.
"I had Hayden's record at the back of my mind when I started my innings today. I am a bit disappointed at missing Hayden's record. But I will settle for a 300 any day. This innings will definitely give me a lot of confidence. It is particularly satisfying because no Indian had done it till now," he said.
The flamboyant opener broke teammate VVS Laxman's record of 281, the highest individual score in Tests by an Indian, which he established against Australia at the Eden Gardens in Kolkata in 2001.
Sehwag said his idol Sachin Tendulkar, who hit up an unbeaten 194, was reminding him not to play loose shots.
"I just told myself that I should play for as long as possible. My first objective in the morning was to reach 300. Tendulkar kept reminding me that I should not play any loose stroke. I was having my own plans that was to keep attacking and play my strokes but Tendulkar asked me to cool down. He pointed to me that a 300 does not come easily for every batsman and it is a lifetime opportunity and I should not let the chance go," he said.
Asked whom would he dedicate the triple century to having dedicated his double ton to his parents and would-be wife, pat came the answer "to the same people".
"People have talked about my inability to hit the big scores. But now that I have scored 300 there is going to be more expectations," he said.
Sehwag also said he was determined to carry on the good work for the team when play started on Monday morning.
"I slept well in the night, in the morning I told myself that I should try and get the triple century. I am happy that I could achieve that.
Sehwag's milestone earned him a lot of praise from his teammates with batting maestro Sachin Tendulkar describing it as a "fabulous knock".
"It is a fabulous knock, I hope he will continue playing in this way. At last, we have an Indian in the 300 club, it was high time. We feel proud of Veeru," he said.
Captain Sourav Ganguly, who had to sit out the first Test because of a troubled back, said "it was an outstanding innings which settled the issue about opening the innings. He has done a phenomenal job at the top of the order".
Rahul Dravid, leading the team in Ganguly's absence in this Test, said, "it was a fantastic effort, full of breathtaking shots -- it was a privilege to watch the innings".
Laxman, whose record fell by the wayside, was equally effusive in his praise of Sehwag. "I am extremely pleased that Veeru has become the highest Test run-scorer for India. I always thought that he would do it because he scores so fast. But I am somewhat disappointed that he did not go past Hayden's record," Laxman said.