I live in Sweden and I have a national ID card. Can I use it when traveling to Iceland? In security checks, gate checks etc? Or, do I need to show my passport everytime?
Per the Nordic Passport Union, Nordic citizens enjoy full right of abode in each other's countries (incl. the Faroes and Greenland), are immune from deportation, and aren't even legally required to carry proof of citizenship for internal travel. This is flawed these days, stemming from a time when our countries were ethnically homogenous and it was obvious who was/wasn't a Nordic citizen.
Essentially, for all everyday practical purposes, we're not foreigners in each other's countries.
As such, we can use any official photo ID (e.g. driving licence) to travel to Iceland from another Nordic country.
- Connecting in a non-Schengen state
- Flying Oslo ⇄ Stockholm on Ethiopian Airlines, Copenhagen ⇄ Helsinki on Sichuan Airlines, or Reykjavik ⇄ Helsinki on Juneyao Airlines
then one will clear the external Schengen border, whereby proof of citizenship (passport or national ID card) is needed.
So yes, a national ID card is sufficient under any and all circumstances.
Yes, Iceland is closely associated with the EU and all EU citizens can go there with their ID card (if they have one, of course). It's also part of the Schengen area, which means there is no systematic border checks and it's even possible you won't have to show your ID at all.
Depending on airline policy and on the airport, you might still have to show an ID at some stage but your national ID card will be enough.