Data obtained under the Freedom of Information Act found 18,778 children aged 11 to 18 were admitted to hospital for self harm in 2015/16, the National Society for the Prevention of Cruelty to Children (NSPCC) found.
That figure represents a 14% rise on the 16,416 hospital admissions in 2013/14, the data from England and Wales shows.
Youngsters aged 13 to 17 were most likely to end up in hospital for self-harm, including injuries from cutting, overdosing on pills or burning themselves.
Figures from the Childline helpline run by the NSPCC showed it delivered 18,471 counselling sessions on self-harm last year - roughly 50 a day.
"Knowing hospital beds are full of young people crying out for help should be a real wake-up call to all those that care for the well-being of the younger generation.
"It is vital we confront the fact that an increasing number are struggling to deal with the pressures and demands of modern-day life, to such an extent they are inflicting terrible damage upon themselves.
"It is clear from the thousands of calls Childline receives that we have a nation of deeply unhappy children."
He added: "We know this unhappiness is partly due to the constant pressure they feel, particularly from social media, to have the perfect life or attain a certain image which is often unrealistic.
"They tell us that the need to keep up with friends and the 24/7 nature of technology means they feel they can never escape or switch off, adding to the misery that many feel on a daily basis."
Childline president, Dame Esther Rantzen, added: "It is deeply disturbing that so many children and young people are ending up in hospital because they are injuring themselves so seriously.
Dr Max Davie, assistant officer for health promotion at the Royal College of Paediatrics and Child Health (RCPCH) said early intervention is essential.
He said: "One way of providing this early intervention is for all schools to deliver comprehensive Personal Social Health Economic (PSHE) education, teaching children about emotional well-being and addressing challenging mental health issues such as eating disorders, self-harm and suicide in addition to other important topics like positive relationships, sex education and the dangers of drug and alcohol abuse."
A Department of Health spokesman said: "We want children with mental health problems to get the help they need.
"That's why we are investing £1.4bn to help every area in the country transform services for young people with all mental health conditions, including self-harm."
:: Contact Childline on
0800 1111
