Reebok will have exclusive rights to provide the club's first team, reserve, academy and replica kits, along with a range of sports, leisure and fashion wear.
The agreement takes effect from season 2003-2004.
"Reebok are one of the key players within the sporting goods industry and are renowned for producing technical and quality products," says West Ham United commercial director, Stuart Ryan.
"They have produced some excellent playing kits and training wear and this design expertise has been utilized in producing next year's Hammers range.
"We are pleased to have a global brand the size of Reebok involved with the club, and the partnership agreement will further enhance Reebok's involvement within the UK football market."
Reebok UK managing director, Dave Singleton, explains: "As a dynamic and progressive leader within the industry, Reebok seized the opportunity to form an alliance with West Ham United that will jointly develop strategic initiatives within the UK and international markets."
Chairman Terence Brown adds: "We are excited about this partnership - and the opportunities that it presents both parties."