tmux does not natively support XWindow’s clipboard. With two lines in .tmux.conf, you can configure two keys to send and retrieve clipboard content.

Traditionally, applications use the PRIMARY selection, which uses the mouse selection for copying and pasting with the middle mouse button. However, this is becoming less common, so I’ll configure the CLIPBOARD selection used by most modern browsers, applications, and Emacs.

Add the following lines to .tmux.conf:

# move x clipboard into tmux paste buffer
bind < run "xsel -ob | tmux load-buffer - ; tmux paste-buffer "
# move tmux copy buffer into x clipboard
bind > run "( tmux show-buffer | xsel -bi ) && tmux display-message \"ok!\""

The first line sets C-b < in tmux to retrieve the X clipboard contents and paste them. The second line sets C-b > to send the current tmux buffer to the X clipboard for pasting into another application.

If you would still like to use the PRIMARY selection, you can drop the b options from xsel, using xsel -o and xsel -i respectively.