The poll, carried out by the mental health charity Young Minds, also suggests a third of 11 to 14-year-olds had watched porn on a mobile device.
The survey questioned 2,000 people aged 11 to 25.
Young Minds said the results show porn is having a "damaging impact" on young people.
The survey, published on Safer Internet Day, suggests many people who watch porn think it has affected their relationships.
Overall, 13% of those questioned said watching porn "definitely" had an effect.
A further 12% answered "occasionally" and 14% said their relationship was "slightly" affected.
The charity suggested the rise in popularity of smartphones and tablets has made it easier to access porn.
"It is something teenagers are always going to encounter but never like this," said Lucie Russell, Director of Campaigns at Young Minds.
"Anyone can do it, anywhere, any time," she added.
Have you ever gone online (on your mobile or tablet) and watched any pornographic content?
The poll questioned 2,000 children and young people aged 11 to 25.
Other findings of the survey include:
Slightly more males questioned had accessed porn on a smartphone or tablet with 59% saying they had, compared to 44% of females.
Younger people questioned were more likely to watch porn with a group of friends - 29% of 15 to 18-year-olds had done so.
The survey also asked how watching porn made people feel, with the most common answer being "excited".
However, a quarter of people questioned said they "didn't feel anything".
Jake, who did not want to give his real name, is a 20-year-old student and describes himself as a "regular user" of pornography.
He said he started watching porn online aged 13 and does not think it has had any negative effects on him.
"You know that most of your mates will watch pornography and I think that's a completely normal habit.
"If you get along with your friends there's no reason why you wouldn't want to talk about it. You can have a good laugh talking about it."
Consultant child and adolescent psychiatrist, Dr Eileen Vizard, said that repeated viewing of porn can have a damaging effect.
"With children, whose brains are still developing, there could be long lasting effects on the way their brains function."
She added that young people watching porn can often turn to more extreme types of material.
"They tend to escalate the seriousness of what they want to look at, sexual material that involves coercive acts towards women or maybe children."
Young Minds says children in primary school should be taught how to navigate the internet safely.
"We need to be talking about pornography in the classroom," said Ms Russell.
Radio 1 Newsbeat and BBC Radio 5 live will broadcast a special joint programme about online safety on 11 February between 12:30 and 13:00 GMT.
Follow @BBCNewsbeat on Twitter

Eleven passengers and three crew were on Friday morning's Eastern Airways flight T3 722, which safely returned to Norwich at 07:26.
The airline said there was a smell but no smoke.
The passengers were transferred to another flight so they could continue their journey to Aberdeen.
Eastern Airways said the Saab 2000 aircraft had returned to Norwich as a precautionary measure.
A spokesman said: "The captain operating this morning's T3 722 flight from Norwich to Aberdeen made a precautionary return to Norwich to have the aircraft checked following an unusual odour in the cockpit.
"The airport fire service was initially put on standby, but was stood down on arrival. Our engineers will be carrying out an inspection of the aircraft.
"We apologise to passengers for the inconvenience, but the safety of our passengers and crew is our primary concern."
Around half of 15 to 17-year-olds have accessed pornography on a smartphone or tablet, according to a survey seen by Newsbeat.
[NEXT_CONCEPT]
A plane headed for Aberdeen was turned back to Norwich Airport after the crew reported smelling an "unusual odour" in the cockpit.