Flintoff will help punters with cricket tips via William Hill's podcast service and promote the betting firm through a range of in-store and other brand activity.
Kristof Fahy, William Hill chief marketing officer, hailed the signing of Flintoff, who joins existing brand ambassadors Robbie Savage and AP McCoy, as "a huge coup", adding that the former cricketer "perfectly embodies what we want from our ambassadors".
Fahy said Flintoff had been brought in as it gears up for two Ashes series in the space of six months, because he is "opinionated, deeply passionate about the game and instantly recognisable".
England's first match in the test series against arch rivals Australia will start at Nottingham's
Flintoff shot to fame for his seminal role in England's 2005 Ashes triumph, in which he became the first Englishman to take more than 20 wickets and score more than 400 runs in a single Ashes series, and was later named BBC 'Sports Personality of the Year'.
Flintoff, who has nearly 1.2 million Twitter followers, said: "This is a great time to join William Hill – it is a double Ashes year and it could be one of the best years in English cricket history."