But it took six rounds of voting, including elimination rounds, before the Council were able to reach an absolute majority in its favour.
Other contenders were Brussels, Moscow, Rome, Berlin and Budapest - the latter two posing the biggest threat to Helsinki in the final round of voting.
Following the announcement of the winner, IAAF president Lamine Diack said: "First of all, I would like to congratulate all of the contenders. We have shown with the bidding process that concluded here in Nairobi that there is tremendous worldwide interest in our sport.
"It was not easy for the Council to select one candidate and it took more than 40 minutes. However, I am now confident that we have chosen the city that offered the best conditions for these World Championships.
"We believe that Helsinki will guarantee a full stadium throughout the competition, as athletics is the number one sport in Finland."
"You have our word that the 2005 World Championships will be a great success. The whole of Finnish society will be supporting this from tomorrow."
London lost the rights to host the Games after the UK government backed out of plans to build an athletics stadium in Picketts Lock, north London.
