In addition to doing media and marketing work as the face of the company, Dixon will work closely with the LA Fitness board devising new dance and exercise routines and advising on health and fitness issues.
Her remuneration will be based on the success of the chain over the course of the deal, with the highest amount she could earn being three times bigger than the lowest, according to one source.
Martin Long, chief executive of LA Fitness, said: "Alesha will bring a fresh, contemporary eye to the entire LA Fitness business. We look forward to working with her to develop new classes and products that our current members will enjoy, and that capture the imagination of potential members."
LA Fitness is currently undergoing a £30m repositioning, including the refurbishment of all 83 clubs and the launch of new products for its 230,000 members.
Dixon said she decided to work with the chain "because they have a great product that's accessible to everyone. I'm really looking forward to helping them develop the business by working alongside the LA board to advise on products and class routines."
It is not the first brand tie-up for Dixon, who went from being in girl band Mis-Teeq to winning 'Strictly Come Dancing'.
She joined the programme as a judge this year, a decision marred due to the controversial axing of Arlene Phillips, in addition to developing her solo singing career.
She has also supported her touring with promotional tie-ins with Toblerone and the Nectar loyalty scheme, while back in January 2008 Dixon's 'For You I Will' music video was funded by Ford Focus.