The club's 13-year deal with Nike, worth 303 million pounds ($502 million), expires at the end of the 2014-15 season.
Nike didn't agree to a new contract during last year's exclusivity period but would have the right to match a rival offer.
Adidas, Warrior and Puma are among those interested in becoming the club's new supplier, several British newspapers reported Tuesday.
"We currently are in discussions with multiple parties regarding our global retail, apparel and product licensing business starting in the 2015-16 season," United said in a statement. "This process is a routine practice used for our global and regional sponsorship deals and helps us maximize the value/structure of the club's partnerships."
United has one of the most lucrative brands in sport and topped English soccer last season in revenue at 363.2 million pounds ($602 million).
The club, which is listed on the New York Stock Exchange, will receive $559 million under a seven-year shirt sponsorship that starts next season with General Motors Co.'s Chevrolet division.