The December flat fixtures have been moved to Wolverhampton and Lingfield, with the former set for a 14-race afternoon and evening card on December 15.
Its National Hunt fixture on December 4 will be offered to other jumps courses.
"We had just short of 50mm (two inches) of rain on Saturday and going into Sunday, and we've had a further 10 or more millimetres since," said clerk of the course Roderick Duncan.
"This, added to the high water levels alongside the River Trent which was almost at the top of its banks, has caused widespread flooding in the area."
Southwell, which has an all-weather surface and therefore hosts racing all year round - as well as boasting a golf course - closed for five months in 2007 due to flooding.
Owners Arena Leisure, who are insured, said that the previous closure cost around £6m.
"Back in 2007 the Environment Agency said it was something that happened once every 50 years, but unfortunately it's only been five years," said Arena spokesman Nathan Corden.
"We are an all-weather track. I know that's ironic for some people that we can't race, but we are under water. This is our busiest time of the year where we normally come into our own."
The racecourse, which has also been forced to cancel a beer festival, hopes to reopen in January.