Lars Oscar Fonbæk (6 July 1887 – 7 September 1965) was a Norwegian long-distance runner.
He was born in Ullensaker, but represented the club IF Torodd in Oslo.
In 1908 he won a bronze medal in the 1500 metres and a silver medal in the 5000 metres.
In 1909 he set two unofficial Norwegian records in the 3000 metres and the 1 hour run.
He competed in the marathon at the 1912 Summer Olympics, but did not finish the race.
His personal best marathon time was 2:44.30 hours, achieved in 1912.