Britton has picked the all new Auris as her car of choice as she prepares for the World Cross Country Championships on March 24.
In December 2012, Britton became the first woman to successfully defend the European Cross Country Championship title.
Last month she produced an outstanding performance to win the 2013 Great Edinburgh Cross Country.
Ian Corbett, Marketing Operations Manager for Toyota Ireland, said: "We, at Toyota Ireland, are delighted to announce this brand ambassador deal with Fionnuala Britton.
"When it comes to athlete sponsorship, our aim is to partner with people that show exceptionally high standards and dedication to their craft.
"Fionnuala has been the outstanding performer in Irish athletics in recent years and we look forward to supporting her through the World Cross Country Championships and beyond in her quest for qualification to the 2016 Olympics in Rio."
Britton said: "I am delighted to become an ambassador for Toyota here in Ireland.
"Training mostly from Dublin and with the typical inclement weather it is fantastic to have reliable wheels to take me to and from training.
"I love the look of the Auris and I am greatly impressed with the car's fuel efficiency and comfort."
