As of February 25, 2026, U.S. citizens do not need a traditional "visa" for short stays in London (up to six months), but they must have a valid Electronic Travel Authorisation (ETA). This is a new mandatory digital requirement for all visitors from visa-exempt countries. The UK ETA costs £16 (approx. $20) and is valid for two years for multiple entries. You should apply at least three days before your flight via the official UK government website or app. Without a confirmed ETA linked to your passport, you will be denied boarding by your airline. This system is similar to the U.S. ESTA or the European ETIAS and is designed to pre-screen travelers for security purposes. Once you have your ETA, you can still use the "eGates" at Heathrow or Gatwick for a seamless entry, provided you are over the age of 12 and have a biometric passport.