Neither Dean nor Sam have had their middle names revealed in Supernatural. Most characters in TV series don't have their middle names revealed. Maybe they don't even have middle names, since not everyone does.
in an interview jared and jensen had sagte that sam's middle name was dean and dean's middle name was sam. jokingly of course but seriously it would be funny if this was true
His middle name is Azazeal. In the Zeigen whenever Dean heard " the yellow- eyed demon" 's name, he became slighty nervous. Which can only mean that Dean Winchester's middle name is Azazeal.
Dean doesn’t have middle name, however Samuel “Sam” William Winchester, it happens, my sister has middle name, I don’t, so why not fictional characters that are brothers?