Traveling with two passports - which one to use?

I would just like to add a small detail to the otherwise excellent answers of Richard Gadsden and Omar Kooheji:

• some countries do not have actually immigration exit checks performed by immigration officers at airports, they rely on airlines to provide the relevant information

• this is for instance the case in the US, as well as in the UK

• in this case, when travelling from that country to your other passport's country, you do indeed show you destination country's passport, but they may ask for the other passport

• this was for instance the case in the US before the current Visa Waiver Program was set up: on arrival, foreigners would get a part of the I-94W form stapled into their passport, and the airline was supposed to collect it to return to immigration authorities. If you entered with an US passport, and then exited with another passport, they would be missing the form in the passport, so would need to see your US passport to clear things out.

• don't know how they sort it out nowadays (they probably keep track of your APIN data from both legs), but in the US case at least I don't believe I have been asked about it in a while

Conclusion: don't be surprised if the check-in agents ask to see the other passport (the one from the current country). This will most certainly depend on the pair of countries, of course.

There are usually multiple places where you'll have to show passports and things can get a bit tricky. Obviously you'll use the passport of the country whose passport you are at. But often you want to show the passport of the destination country at check in to show the airline company that you have the appropriate visas/residency.

So for example for a trip from Germany to the US the sequence is as follows:

• Check in with the US passport (Airline will actually send that info to the US)
• Use German passport for passport control when exiting Germany
• Enter US with US passport

Show your Saudi passport to the Saudi Immigration, show your British passport to UK immigration. If the airline asks if you have a Visa for the UK show them your UK passport.

Do not show Saudi Immigration your UK passport. As I understand you may get into trouble if you do.

I do this all the time and it's never been an issue for me. it used to be that you could get a certificate of right of abode in your Saudi passport that acted like a permanent visa meaning that you didn't need the UK passport, but they don't issue these any more.

There are three different (groups of) people you show your passport to on an international flight.

1. Immigration exit on the country you are leaving
2. The airline you're flying with
3. Immigration entry in the country you are entering

The rules for all dual nationals flying between their two countries are:

• Always show your passport for a country to that country's officials
• Always show the passport for your destination to the airline.

So, flying from the UK to Saudi, you show your Saudi passport at check-in (airline), then your UK passport to immigration exit (UK immigration), and then your Saudi passport again at the gate (airline). When you land, you show your Saudi passport to immigration entry (Saudi immigration).

Flying from Saudi to the UK, you show your UK passport at check-in (airline), then your Saudi passport to immigration exit (Saudi immigration), and then your UK passport again at the gate (airline). When you land, you show your UK passport to immigration entry (UK immigration).

You need a visa for neither country because you are a citizen of each.

When you enter a country, in order not to need a visa, you show them the passport of that country.

In your case, when arriving to Saudi Arabia, you show them the Saudi passport. When arriving to the UK, say after you get back, you show them the UK one. You obviously need to have both with you but you always show one at any given time.

It is possible there are legal exceptions, but this works in most parts of the world. For Saudi Arabia, which gives no visas to tourists, you pretty much have no choice.