Reitman was in Rome to accept the award, along with screenwriter Diablo Cody.
The lucrative award also comes with $287,700 (Euros 200,000).
'It is terrifying to bring your film to another culture where they speak another language, and this is the first place it has played outside of North America. They say you really don't know another culture until you can make them laugh,' Reitman said as he accepted the award.
All of the fest's winners were selected by Rome's popular jury, made up of 50 European and Italian filmgoers presided over by Oscar winning director Danis Tanovic.
Rome's special jury award went to Abolfazl Jalili's Hafez, an Iran-Japan co-production that held its world premier at Rome. The jury cited the film's ability to 'pit human strength against tradition and fanaticism which connects strongly to current political issues.'
Best actor honors went to Rade Serbedzija for Fugitive Pieces, directed by Jeremy Podeswa.
China's Jiang Wenli took best actress for As the Spring Comes (Li Chun) by Gu Chang-wei, which also had its world premiere in Rome.
In other official prizes, the Alice in the City sidebar children's juries named two best film prizes, each coming with $35,900 (Euros 25,000) in prize money. They were Canvas by Joseph Greco (K12) and Meet Mr. Daddy (Noonbushin Narae) by Kwang Su Park.
Extra - dedicated to documentary and non traditional works, awarded the non-official $28,700 (Euros 20,000) Cult prize for Best Documentary Film to Forbidden Lie$ by Anna Broinowski.
Rome Film Fest 2007 winners
Marco Aurelio Award for Best Film - Juno by Jason Reitman
Special Jury Award - Hafez by Abolfazl Jalili
Marco Aurelio Award for Best Actress - Jiang Wenli for And the Spring
Comes (Li Chun) by Gu Chang-wei
Marco Aurelio Award for Best Actor - Rade Serbedzija for Fugitive
Pieces by Jeremy Podeswa
Alice in the City
K12 Prize for Best Film - Canvas by Joseph Greco
Young Adult Prize for Best Film - Meet Mr. Daddy (Noonbushin Narae) by Kwang Su Park
Paolo Ungari/Unicef Prize - (Alice in the City Book Section) Ragazzi
di Camorra (Kids of the Camorra) by Pina Varriale
Non official collateral Prizes:
Fastweb prize for the Premiere section -Into the Wild by Sean Penn; Special mention to Days and Clouds (Giornie Nuvole) by Silvio Soldini
L.A.R.A For Best Italian performer to Giuseppe Battiston for The Right
Distance (La Giusta Distanza) by Carlo Mazzacurati
Cult Prize for Best documentary presented in Extra: Forbidden Lie$ by
Enel Cuore Prize for Best Social Documentary Film Presented in the
Extra Section: War/Dance by Sean Fine and Andrea Nix-Fine
Mini Prize for Best European Project at the New Cinema Network:
Slawomir Fabicki for the project Bonobo Jingo
L'Oreal Paris for best make-up: Valentina Lodovini actress in The
Right Distance (La Giusta Distanza) by Carlo Mazzacurati.