Do I need a US visa to transit (or layover) through an American airport?
In Europe usually you don't need to go through immigration to change planes (unless you are entering/exiting Schengen). Is it the case in the US as well? May I catch a connecting flight on one of the US airports if I don't have US visa?
Shorter version: **Yes**, you need either a visa or visa waiver (ESTA). Getting the latter can actually take *more* trouble/money than a proper visa to some other countries (so it's not much of a "waiver"). And **yes**, you need it even if you merely change planes in the US. Yes, this sucks.
Unlike many other countries, US airports do not have any form of physical immigration controls when you are departing the country on an international flight. In fact, in most airports there isn't even a concept of an "International" terminal/gate, with the same gates frequently being used for international flights and domestic flights.
As a result of this they can't enforce the concept of a 'transit' passenger - once you're in the departure area, even if you got there on the pretext of catching another international flight out of the country a few hours later, there's nothing to stop you boarding a different domestic flight, or even simply walking out of the airport!
This means that ALL passengers arriving on international flights in the US must have the legal right to enter the US; i.e, you must either be a US citizen/green card holder, be from a country that does not have any visa requirements (such as Canada or Bermuda), be from a country that is a part of the US Visa Waiver Program and have a valid ESTA, or have a US Visa.
If you require a Visa then there is a "Transit Visa" (C) available which is normally a little easier to obtain than a normal visa, but you'll still need to go through the full visa application process, including showing proof that you intend to leave the US (almost) immediately.
I'll second this. I actually have walked out from the international departure gate to an area outside--the plane was very late and I wanted food other than what was available in the terminal. There was no check of any kind on the way out and coming back there was nothing but the normal security you would find in any airport.
+1 I flew into the USA from Fiji, landed in LA with New York as my final destination. But I had to go through immigration control in LA, collect my checked-in luggage and check in again to board my flight to NY.
I'm slightly confused. How to they perform immigration control in these cases i.e how do they "know" if I have a visa or not? Is this only the case for transit flights or is this the case for arrivals as well?
@AdityaSomani All arrivals in the US must go through immigration, regardless of whether you are transiting or arriving. There is no way to get to a departing flight from the area where incoming flights arrive except via immigration.
@Doc Ahh I understand now. So basically once you leave the arrival after the immigration you MUST have a transit VISA or any other similar document because the departure lounge is unprotected and you can walk out at ease right? I remember what Loren is talking about in a previous comment quite vividly from one of my own experiences in Atlanta as well.