The trains were travelling early on Thursday on a line linking the towns of Hillerød and Kagerup in the North Zealand area of north-east Denmark, with a total of 37 people on board, according to police.
Tim Ole Simonsen of the Greater Copenhagen fire department was unable to say what led to the crash but told Danish TV that all the injured had been taken to hospital by air or by ambulance.
Local mayor Trine Egetved said she was deeply shaken by the accident.
Public broadcaster DR showed images of two yellow and grey trains, both with visible damage to the front, facing each other in a wooded area.
Officials said the trains collided at 06:29 local time (05:29 BST) on Thursday and fire and rescue teams were alerted immediately.
