Emergency services were called after a 37-year-old man was found stabbed outside the store in Western Road, Brighton. Two crews from South East Coast Ambulance Service were sent along with an ambulance car.
The victim was rushed to hospital in a serious condition.
It is believed that he suffered a single stab wound to his chest.
Sussex Police said a 28-year-old man had been arrested on suspicion of committing the attack.
He has been arrested on charges of grievous bodily harm and is being detained in custody.
Montpelier Road was closed temporarily but has since re-opened.