First of all, you must to be updated with your remote.

After that, the command is:

 git branch --merged | grep -v "\*" | grep -v master | grep -v dev | xargs -n 1 -I {} git branch -d {}

Explanation:

• git branch --merged: list only branches merged into HEAD of current branch
• grep -v "\*": remove the current branch from the list (we do not/can not remove it)
• grep -v master: remove master branch from the list
• grep -v dev: remove dev* branch from the list (useful if you are working with GitFlow or develop branch)
• xargs -n 1 -I {} git branch -d {}: we will remove branch per branch (because we set the -n to 1).

If you want to list all branches that this command will delete, remove the last part:

 git branch --merged | grep -v "\*" | grep -v master | grep -v dev

Originally created in @artefactop 's Gist