He has held the position since the operation became a directly-controlled subsidiary in 2002.
During that time Williams has led the growth in Kia’s UK performance –sales are up from less than 13,000 in 2002 to 40,000 in 2005.
Byung Kwang Jang, president of Kia Motors (UK) Limited, said: "Paul led our operation through a remarkable period of growth. He has been fundamental to the brand becoming a major player in the UK market and along with our dealers has dramatically enhanced our reputation for value, reliability and quality."
Williams (45) joined Kia in October 2002 from the Colt Car Company where he was sales and marketing director. Previously he was chief executive at Daihatsu (UK). He has also held posts with Mercedes-Benz, Nissan and Peugeot.
He said: "I am very proud of what we achieved at Kia and I believe that during the last four years we built the foundations of an extremely successful franchise that not only looks to build sales but also seeks to work to enhance dealer profitability and customer satisfaction to the very highest levels.
"This was a difficult decision to take, but as Kia seeks to move on in the UK new car market towards its ultimate goal of 100,000 sales, I think the time is right for me to move in a new direction and for the company to continue its journey without me. It has been an amazing ride for the last four years and I wish Kia and all the team at head office in Weybridge and the dealer network all the very best of luck. I am sure that the focus on providing the best quality vehicles to demanding customers will continue."
Until a successor is found Kia Motors (UK) will be run by its managment board, made up of eight Brits and four Koreans.