South Yorkshire Police said officers were called to London Road and the boy was taken to hospital for treatment of "gunshot-like" injuries.
The road has been closed from the Sitwell Place junction to the Crowther Place turn-off to allow for investigation work.
Senior Investigating Officer, Detective Chief Inspector Emma Knight, said: "This is a horrific incident that has left a young man fighting for his life, and I know the people of our community will be deeply concerned.
"I want to reassure residents that a dedicated team of officers and staff are working tirelessly to understand the circumstances that led to this attack and to identify those responsible."
The force said extra patrols will be deployed in the area in the coming days.
Meanwhile, business owners on London Road have spoken of their shock, with many places forced to close while forensic investigations continue.
Dari Saleh, owner of Marmaris Turkish Restaurant, said: "We work here all day and evening, and then something like this happens. We're scared, our customers, our business, you don't feel safe."
Mr. Saleh, whose restaurant has been open since 2017, said he was worried about the impact on his business.
"People care more about their own safety." If you hear something like this, people won't come."
Abdurrahman Gurner, owner of Bodrum Barber on London Road, said: "Every day it's a bit scary to think that the city is getting worse.
"As a business owner, we wonder if business is going to decrease, are people going to be worried about coming here?"
Reacting to the shooting, Abtisam Mohammed, the Labour MP for Sheffield Central, said he was "deeply shocked and saddened."
He said, "My prayers are with him, his family, and all those affected by this incident, as they are facing a very difficult time."
"I urge anyone with any information to contact South Yorkshire Police and help clean up this incident."
The MP said that violence on the streets would not be tolerated and added: "Every young person in Sheffield should feel safe and confident in their community."
Speaking late Monday night, police said they expected local disruptions to the road network to continue until Tuesday while the investigation continues.
Detective Chief Inspector Knight said: "We need you to work with us and provide any information you have. This is unacceptable on our roads, so we must work together to stop it."