NSPCC Cymru said just two children were reported to North Wales Police in 2013, rising to four in 2014 then 28 in 2015.
Most of those reported - 24 - were aged 15 or under while 10 were 16 or 17.
Des Mannion, head of service at NSPCC Cymru said: "It is very worrying that more children are sharing explicit images of themselves."
He added: "Although the figures in north Wales are relatively small, the fact that there has been a sharp increase in the number of cases reported to police is a concern."
The NSPCC wants children to be warned about the dangers of sexting through compulsory online safety lessons and wants to see the next Welsh Government implement a plan for this.
While the majority of these types of images were shared via mobile phones, they were also sent using apps such as Snapchat, Instagram and Facebook.
Mr Mannion added: "We are very concerned that children are putting themselves at risk of exploitation or bullying in this way.
"Children need to be taught about the dangers of sexting and how the consequences can be devastating if they lose control of an image."
In February, police in England and Wales began producing new guidelines designed to avoid "criminalising" children caught sending indecent images to each other.
A rise in under-18s being reported to the police for distributing indecent images of children has raised concerns over 'sexting'.