Fast-thinking Adam Hurrell, from Chapeltown, jumped into the water at Hillsborough Leisure Centre after spotting the teenager lying on the pool floor.
The lifeguard dragged the unconscious teenager on to the pool side and resuscitated him before paramedics took over.
Adam has been a lifeguard for three years but said he has never had to deal with such a serious incident.
The 22-year-old said he had to think on his feet and put his training into practice.
Despite the drama, the hero lifeguard was back at the pool working the next day.
“It happened as quick as a flash – it seemed to be all over as quick as I jumped in,” Adam said.
“I saw this lad underneath the water and, at first, nothing seemed amiss because he wasn’t in that deep. But I saw blood coming from his mouth and I jumped straight in.
“I pulled him out and another lifeguard sounded the alarm – I started with the CPR and the lad came round after about a minute and a half and then the ambulance arrived.”
Colleagues and friends have praised his actions but the lifeguard said he was just doing what he has trained to do.
“I’m no hero – I was just doing my job,” Adam said.
“Some people might say that but it’s what I do on my training and I just did it.
“I’ve had loads of compliments and ‘well done’ messages and I am pleased we managed to bring him around.
“It’s really nice to hear people saying you’ve done a good job.
“People can be in the job for ages and not have to deal with something like this – you have to be alert.”
Duty manager Lauren Hetherington, who trained Adam in his emergency response work, said: “Adam is a hero.
“Anyone who has to pull someone out in those circumstances deserves special credit. I couldn’t ask anymore from him in what he did.
“He deserves the credit – he’s a great lad.
“You go into a shift and you don’t know what to expect. You have to be on your toes even when it doesn’t seem like anything is going on.
“We often get quite a bit of negativity from people saying lifeguards don’t do anything. We’ve been called lazy and the ‘fun police’ for telling people to stop running around the pool for instance.
“I think people swimming on that day were quite stunned that Adam and people who work here are trained to deal with something like what happened.”
Staff at Hillsborough have not received any update on how the teenager is doing.
“We would love to hear from him to see if he’s doing okay,” Lauren added. “Even if it’s just a phone call to say so then it would put our minds at rest.”