Earlier during the day, international election monitors hailed the election as an "impressive" display of democracy and urged the nation's political leaders to ensure a peaceful transition of power.
"Yesterday's vote was an impressive display of democratic elections. For everyone in Ukraine, this election was a victory," the observers, headed by the Organisation for Security and Co-operation in Europe (OSCE), said in a statement.
"It is now time for the country's political leaders to listen to the people's verdict and make sure that the transition of power is peaceful and constructive."
The OSCE verdict appeared to be tantamount to a call by the international community for Tymoshenko, to accept the fight was over.
The election commission projected the turnout among Ukraine's 37 million voters at about 70 per cent, 3.2 percentage points higher than the January 17 first-round vote in which 18 candidates competed.
The result marks a remarkable comeback by Yanukovych, a 59-year-old former prime minister, who was disgraced in 2004 by the "Orange Revolution" mass street protests which Tymoshenko co-led.
Yanukovych's election that year, in a poll deemed to have been rigged, was quashed by a court and he lost a third round of voting to Viktor Yushchenko, the other leader of the Orange Revolution and Ukraine's outgoing president.
Neave Barker, Al Jazeera's correspondent in Kiev, the Ukrainian capital, said Yanukovych had learnt a lot in the last five years.
"Back then it was very much felt that he alienated voters in the west of the country - voters who looked towards the European Union for a future for the country - by mentioning the need to maintain strong ties with Russia," Barker said.
"This time round he has refused to speak at all about anything that may be vaguely controversial.
"In the words of one analyst, he's played a secure campaigning game plan, making sure he doesn't put anyone off."
|Yanukovych is characterised as being closer to Russia than Tymoshenko [AFP]
The close result reflects broader divisions in the former Soviet republic of 46 million people, divided almost equally between a Russian-leaning east and south and a Western-friendly centre and west.
Both candidates have said they want to integrate with Europe while improving ties with Moscow, although Tymoshenko is seen as more enthusiastic about the European Union and Yanukovych is characterised as being closer to Russia.
The election was closely watched in Russia, Ukraine's former Soviet master, but state-controlled media avoided taking sides.
Apparently keen to avoid repeating Russia's 2004 gaffe of prematurely congratulating Yanukovych, there has been no official comment from the Kremlin.