Five people who were on board the training aircraft from RAF Valley on Anglesey nearby have been reported safe and did not need hospital treatment.
An air ambulance was sent to the scene, along with a coastguard helicopter.
A UK Ministry of Defence (MoD) spokesperson said the 'Griffin' training helicopter "safely completed a precautionary landing in Snowdonia" following a technical issue.
"Everyone on board exited safely, subsequently the aircraft caught fire," he confirmed.
The UK Maritime and Coastguard Agency said it was called in the afternoon to reports by the air ambulance of a helicopter crash west of Snowdon, the highest mountain range in Wales.