The woman, in her early 20s, was rescued by RNLI lifeguards who were on duty for the first day of the season at Newquay's Fistral beach.
Lewis Timson, lifeguard supervisor, said the surfer's longboard leash snapped while she was competing in the Natty Surf Jam one-day event.
He described conditions at sea as "pretty wild".
The "distressed" woman, who is thought to be from Newquay, was taken ashore, but was not believed to need hospital treatment.
The festival was arranged in memory of 25-year-old local surfer Nathaniel Loney who died after battling severe depression and bi-polar disorder.
The money raised is being given to the Invictus Trust, a charity which supports young people with mental health issues.
The rescue highlights statistics from the RNLI, which show a big increase in the number of incidents around the South West.
Last year, beach lifeguards dealt with more than 11,000 incidents, while lifeboat crews launched 1,614 times - a 35% increase from the previous year.
There will be full-time lifeguard cover at Fistral and Croyde in Devon until the end of the season, with a number of other beaches in Devon and Cornwall patrolled during the Easter school holiday period.
A woman has been rescued from the sea after getting into difficulty during a charity surfing festival in Cornwall.