A U.S. Green Card (Permanent Resident Card) is not a valid travel document for entry into the United Kingdom. The UK border authorities determine your entry requirements based solely on the passport you hold, not your U.S. residency status. If your passport is from a country that requires a visa for the UK (like India, China, or many African nations), you must apply for a Standard Visitor Visa in advance, even if you have lived in the U.S. for decades. If your passport is from a "visa-exempt" country (like Japan, Canada, or EU nations), you may still be affected by the new Electronic Travel Authorisation (ETA) system fully enforced as of February 2026, which requires all visa-free travelers to obtain digital permission before boarding their flight. The only role your Green Card plays is providing "proof of ties" to your home country (the U.S.) during a visa application, which can help demonstrate that you intend to leave the UK after your visit. Always use the UK government's "Check if you need a UK visa" tool to confirm your specific status before booking.