Can I use a security key such as YubiKey to sign into Google on Safari?
First, I assume you are using YubiKey 4 with U2F support.

Safari does not support U2F natively, but if you have Safari 10, you can add the plugin Safari-FIDO-U2F available from blahgeek on GitHub.

The author confirms that the plugin works with some of the websites (with some hacking required to show up to the server as Chrome):


  
  Github Account Two-factor authentication
  Dropbox Account Security
  Fastmail
  


but also warns it does not work with Google Account (although it does with Google's demo site).

So for now the answer is you cannot use U2F to login to Google, but it might change in future.