BlueIce is a prescribed app - meaning access must be given through medical professionals - and is designed to be used alongside traditional face-to-face therapies.
It was developed by clinical psychologist Professor Paul Stallard, of the University of Bath, in conjunction with patient groups.
A series of papers published in the Journal of Medical Internet Research suggest that it could help to tackle self-harm in young people.
Prof Stallard, who is head of psychological therapies for Oxford Health NHS Foundation Trust, said the idea for the app came through his work with child and adolescent mental health services (CAMHS).
"Many of the young people I was working with were self-harming but nearly all had their mobile phone close by," he said.
"Our young people's participation group at Oxford Health NHS Foundation Trust thought that a smartphone app could be a way of helping at times of distress, and with their input we produced BlueIce.
"BlueIce is a prescribed app to be used alongside traditional face-to-face appointments with a child and adolescent worker.
"It helps the young person to monitor and manage their unpleasant emotions and to find alternative ways of coping.
"Feedback from young users has been overwhelmingly positive, and there's a huge potential for it to make a difference to young lives across the UK and internationally."
The name BlueIce refers to low mood and in case of emergency (ICE).
It is included on the NHS Apps Library, which contains apps that have gone through clinical and technical reviews.
BlueIce has a mood wheel for young people to track their mood each day, adding notes on how they are feeling and what they are doing.
If low mood is reported, the users is automatically routed to a mood lifting section that includes activities to reduce distress.
Options include ideas from Cognitive Behaviour Therapy (CBT) and Dialectical Behaviour Therapy (DBT), as well as personalised music and photo libraries and mindfulness guides.
The app can also take users to emergency contacts such as Childline and the 111 service.
Prof Stallard analysed the impact of 12 weeks of using the app on a group of 40 young people aged between 12 and 17.
He found that 73% of those involved either stopped or reduced their self-harming as a result of the app.
BlueIce is currently being used by CAMHS services in Bath, North East Somserset, Buckinghamshire, Bedfordshire, Oxfordshire and Wiltshire.
It is being evaluated in a randomised controlled trial undertaken across Oxford Health NHS Foundation Trust and at the Murdoch Children's Research Institute in Melbourne.
In September, Prof Stallard will start a trial to see if BlueIce reduces the number of young people taken to A&E.