Bell took over as League One Vale's development squad boss in June, but he has now decided to drop down two divisions to become Coleman's assistant manager.
"I have taken up the opportunity to work alongside John Coleman at Southport. I worked with John over a 17 or 18-year period and we are very close and work quite well together," Bell told Vale's official website.
Vale manager Micky Adams added: "Jimmy has only been here for a short while but he has made a big impact and I would like to wish him well in the future.
"He did a very good job in the six months he was at Port Vale and I am sure he will continue to do a good job with John Coleman at Southport."
Southport are currently in the bottom half of the Conference table, having taken 21 points from their opening 17 matches this season.
Coleman, who replaced Alan Wright as Southport manager last week, will take charge of the Sandgrounders for the first time on December 21 when his new team visit Salisbury City.