How do I lock a Telegram chat on iPhone?

If you are using Telegram's Secret Chats for the end-to-end encryption it provides, then you probably also want to guard your chats from anybody who might pick up your phone. You can do this by opening the Telegram app and heading to Settings -> Privacy and Security -> Passcode & Face ID and tapping Turn Passcode On. You'll then be asked to enter a six digit PIN and confirm it.

Now you can choose to turn on Face ID and set an auto-lock time if you wish.

When you return to your chats you will see a small lock icon next to "Chats" at the top. Tap it to lock your chats. Telegram will be locked now until you type in your PIN or use Face ID. You can get rid of the lock in the same place you set it in Settings, but you'll need your passcode/Face ID to turn it off.