Serifovic's powerful ballad Molitva had been second-favourite with bookmakers in the run-up to the competition.
She scored 268 points, beating Ukrainian drag queen Verka Serdyuchka into second place. Russia was third, with Turkey fourth and Bulgaria fifth.
British act Scooch managed only 19 points with their song Flying the Flag.
EUROVISION TOP FIVE 1. Serbia (268 points) 2. Ukraine (235) 3. Russia (207) 4. Turkey (163) 5. Bulgaria (157)
Twelve of these came from Malta, which placed the UK entry top in its voting, but there was little recognition from other countries.
The UK tied with France one position above the bottom in the results table, with Irish folk group Dervish scoring only five points and coming last overall.
It meant that Scooch narrowly avoided finishing 24th out of 24, as British act Gemini had in 2003 - famously scoring "nul points" - with the song Cry Baby.
But Scooch were characteristically undeterred by the result.
"To be honest, this experience has been one in a million," said singer Caroline Barnes.
"But I have to say I laughed so much I cried at the voting. I'm not gutted. I don't want to say it was expected, but you know what Eurovision is like."
'So proud'
Serbia took first place with its first entry as a solo state at the Eurovision, having declared its union with Montenegro defunct last year.
The Eastern European state will now be charged with hosting the event in 2008.
Following her win, Ms Serifovic told reporters "a new chapter has opened for Serbia".
EUROVISION BOTTOM FIVE 20. Spain (43 points) 21. Lithuania (28) 22=. France (19) 22=. UK (19, pictured) 24. Ireland (5)
"I am so proud. All my success is made by singing."
The competition had been embraced by people in the Finnish capital, which earned the right to stage the event when rock group Lordi won in 2006.
Lordi's song Hard Rock Hallelujah was reprised as the opening number of the show, at Helsinki's largest ice hockey stadium.
Big-screen TVs were erected in the city centre, where fans gathered to watch the ceremony.
Some 350 spin-off events had also been organised as part of Eurovision "fever".
The contest - held since 1956 - had a record 42 entries this year, but 18 were eliminated in qualifying rounds before Saturday's final.
It was broadcast to an estimated global TV audience of 100 million, with the winner selected after a poll in each country involving telephone votes and text messages.
Serbian Marija Šerifović performs the winning song ''Molitva''.

Serbia's entry, Molitva (A Prayer), performed by 23-year-old Marija Šerifović, has won the 2007 Eurovision Song Contest in Helsinki, Finland, with a score of 268 points.

Second place was claimed by the Ukrainian drag queen Verka Serduchka, and third place went to the Russian entry of Serebro. Turkey took the fourth place with ''Shake It Up Shekerim''. Greece with Sarbel's ''Yassou Maria'' came 7th as the first Western-European country, while Ireland took only 5 points, finishing below France and the United Kingdom which shared the second-last place.

16 out of 24 finalists came from Eastern Europe, which caused many Western European countries to doubt the possibility that a country from Western Europe could ever win the final. Although France, the U.K., Spain and Germany are the big sponsors of the festival and are automatically selected for the final round, they all ended up at the bottom of the ranking. The fact that affiliated countries vote for each other (neighbourhood countries such as Scandinavian, Balkan  or ex-Soviet countries) is also an annually returning matter discussed in the media.

A record 42 countries entered the 52nd edition of the Eurovision Song Contest, but 18 didn't make it through the selection rounds. Serbia competed as an independent country for the first time, separate from Montenegro or Bosnia-Herzegovina, and immediately won the competition. This year's edition was the first to host new styles like jazz side by side with traditional ballads and rock. The winning song was also the first non-English song to win since the transsexual artist Dana International won for Israel in 1998.

The competition took place in Finland because last year the Finnish hard rock song 'Hard Rock Hallelujah' by Lordi won the competition. The song kicked off the final in Helsinki. Hosts were Jaana Pelkonen and Mikko Leppilampi

After the victory, many Serbs took to the streets with flags to celebrate the victory. Aleksandar Tijanic, director of RTS state television, was glad that Serbia made the news in a positive way: "I'm so glad it wasn't some war song. Hosting this event in Belgrade next year will mean we have finally crossed into normality." The country is still sometimes associated with the Yugoslav wars which led to the disintegration of Former Yugoslavia.