Fonseca, 46, joins the company from Toyota Spain, where he held the positions of managing director and, vice president and chief operations officer.
He replaces Graham Smith, who after 10 years at the helm at TGB is taking on two new roles within Toyota: as senior vice-president, Toyota Motor Europe, responsible for external and environmental affairs; and as managing director of Toyota Motor Europe at its London office.
Fonseca said: "I am planning to build on the success that Smith has helped the company achieve in the past decade, working to embrace the change the market requires.
"I believe we have to change quicker than our surrounding environment, in order to remain competitive and keep ahead of our rivals. TGB, like all businesses, needs permanent, orderly, yet dynamic and innovative change in order to keep winning.
"I am aware of the difficulties currently facing the UK market, but I am sure that within TGB and its national network of centres, there is the knowledge and determination to maximise opportunities and capitalise on our strengths."
New MD of Toyota GB, Miguel Fonseca