The high-speed Scotrail train left Aberdeen in the early dawn hours before derailing.
The train's driver is believed to be among the dead, but formal identification has yet to take place, the British Transport Police force said. Eyewitnesses said on social media that around 30 emergency vehicles were in the area.
The BBC reported smoke can be seen at the scene and it is not yet clear how many people were injured or how badly.
A train has derailed in Scotland and there are "early reports of serious injuries", according to first minister Nicola Sturgeon.
ScotRail managing director Alex Hynes said the rail operator is working with the emergency services on site as well as the relevant authorities to establish the cause of the incident.
Other UK politicians have also addressed the incident, including the Prime Minister Boris Johnson.
Emergency services have declared a major incident.
A spokesman said: "We're aware of an incident involving a train derailment near Stonehaven".
Local reports said there had been storms and flash flooding in the area overnight.
The train appeared to have been carrying very few passengers.
Officials are describing the crash as a "major incident" with serious injuries.
The Transport Salaried Staffs Association has told the Scotsman that four vehicles of the 06.38 Aberdeen to Glasgow Queen Street service had become derailed.
The worst rail disasters in Britain in recent decades were a 1999 collision between two trains at Ladbroke Grove, in London, in which 31 people died, and a 2001 accident near Selby in Yorkshire, northeast England, in which 10 people died after a auto ran onto the track and was hit by two trains.
The incident took place at Stonehaven in Scotland. Emergency Services at the scene.
Transport Secretary Grant Shapps tweeted: "Just met urgently with @networkrail for a briefing on Stonehaven derailment".
"Thoughts with every single person involved", Mr Bowie, the Tory MP for West Aberdeenshire and Kincardine, said on Twitter.
Labour leader Sir Keir Starmer said: "My thoughts are with everyone involved in the serious incident in Aberdeenshire".