What happens when I delete a Facebook Chat?

When you delete a chat on Facebook Messenger, it will remove that chat and its history from your account and all devices logged in to your account. However, please note that other participant(s) will continue to have a record of the chat and its messages. If you are concerned about specific messages, you can unsend them by first selecting Unsend from the three dot menu next to the message, and then selecting "Unsend for Everyone" to remove it permanently for all participants.