As Campaign reported, the brand split with incumbent Grey London in September, believed to be due to creative differences.
Ginsters, which sells savoury pastries and sandwiches, plans to ramp up its media spend in the coming year, from less than £1 million to £4 million.
Linda Evans, the sales and marketing director for Ginsters, said: "We have some very exciting plans for the Ginsters brand in 2014 so it was important to us that the creative partner we selected shared our vision and aspirations for the brand.
"M&C Saatchi demonstrated their clear ambitions to help us grow the brand still further and we very much look forward to working in partnership with them."
M&C Saatchi has begun work with immediate effect and will produce a new multimedia campaign.
Camilla Harrisson, who was promoted to chief executive of M&C Saatchi last month, said: "Ginsters is an iconic and much loved British brand with a fantastic heritage, big creative ambitions and exciting plans for growth – we are absolutely delighted to be working with them."
Grey created the £6.5 million "man plea" campaign that aimed to encourage men to eat its pasties as a hot meal.
Starcom Mediavest Group, Ginsters’ media agency, was not affected by the change.