Update Dec 22, 2017: We've fixed a recent compatibility issue with Mac OS X 10.7 that caused this error message. We have released a patch that will download and apply automatically for all Mac users. Just restart Steam, and it will update and launch normally.


Original post below:

---

Due to an unexpected compatibility issue between new OS X security features and our recent update, some users may see an error message saying "Failed to load steamui.dylib" which prevents Steam from starting up properly.

If this happens to you after Steam restarts itself for an update, there are two options. First, please try the directions at https://support.steampowered.com/kb_article.php?ref=3134-TIAL-4638 to refresh your local files (including a system restart).

If the problem still continues after a restart, you may need to perform a full re-installation of the Steam application (but not your games). To do this, drag the Steam application to the Trash. You will be prompted to Keep or Discard related files - you should choose KEEP in order to retain your saved games and local downloads. (If you do not see a prompt, then your saved files will not be deleted, as the default behavior is Keep.)

Empty the trash, verify that Steam is no longer an application on your system, and then fully restart your Mac. The restart step is very important, do not skip it!

You can then download and reinstall Steam from this website (Install Steam link at top of page) after the reboot, and the issue should be gone.