He is being treated in hospital for serious burns to his arm and face.
The boy was “deliberately targeted” in the attack, which happened at around 2.15pm on Saturday in Home Bargains at Shrub Hill Retail Park, Tallow Hill.
West Mercia Police have released images of three men they would like to speak to and have appealed for anyone with information about the incident.
Chief Superintendent Mark Travis said: “At this time we are treating this as a deliberate attack on a three-year-old boy.”
He said the incident will “rightly shock the local community” and that police are carrying out a thorough investigation to identify those responsible.
The motive of the attack is unknown.
An ambulance arrived at the scene in eight minutes and crew assessed the boy’s injuries.
After treatment, he was taken to hospital in Worcester. There is no update on his condition.
Officers are working to establish exactly what happened and exactly what the substance involved was.
A 39-year-old man from Wolverhampton has been arrested on suspicion of conspiracy to commit grievous bodily harm.
“The three men in the photo may have information vital to our investigation,” Mr Travis added.
“I am sure someone will recognise them and I would urge anyone with information to contact us as a matter of urgency.”
Worcester City Council leader Marc Bayliss described the attack as “absolutely pure evil”.
He added: “Worcester is not that sort of place. We are a quintessential small English city.
“I have never heard of an acid attack in Worcester so this is absolutely not something we have any experience of.”
Mr Bayliss urged anyone with information to come forward.
“Think if this was a member of your family, an innocent child, a three-year-old probably scarred or damaged for life by this. We need to bring the perpetrators to justice, and quickly.”
:: Anyone with information should call 999 quoting incident 442s of 21 July 2018. Alternatively, call Crimestoppers anonymously on 0800 555 111.