Detective Inspector Paul Shortt, of Nottinghamshire Police, said: "This was an appalling act of violence carried out in full view of members of the public.
Officers believe there was an altercation outside the store between a group of young people before the attack and are appealing for witnesses.
The victim suffered a stab wound to the chest after he was attacked inside the shop in Long Row at about 11.30am on Sunday.
A 17-year-old boy has been stabbed in front of shoppers in a Primark store in Nottingham city centre.
He was taken to hospital with "serious injuries", police said.
