Prior is wicket-keeper for Sussex, and has been called up to the England team to face the West Indies in the first test at Lords on May 17.
This brings the number of Slazenger players in the national side up to four, alongside Paul Collingwood, Ian Bell and Liam Plunkett.
Rashid is a 19-year-old bowler for Yorkshire, who made his debut in July 2006, and is the youngest Slazenger brand ambassador.
Rashid was brought into the England U19 side for the first Test against India at Canterbury in July 2006, and was included in the England A team tour of Bangladesh in February 2007, alongside Prior.
Rashid said: "Slazenger has a track record of manufacturing top quality products, and I am pleased to be in the company of some of the game's most high profile figures as an ambassador for such a high quality brand as Slazenger."
Slazenger's head of marketing, Robin Gibson, said: "Adil and Matthew are exceptional talents and I'm sure both players will be great ambassadors for Slazenger, both in domestic cricket and in the international arena."
The sports clothing company also sponsors the Lashings World XI, a cricketing all-star team. Cricket legends Sir Vivian Richards, Sachan Tendulkar, Shane Warne and Brian Lara have all batted for the side.
Slazenger has also pledged £1.5m to a charity that promotes cricket in state schools, the "Chance to shine" campaign.