The title pretty much says it all: who was the last bearded pope?
A Google search for “bearded pope” comes up with a website which claims that it is Innocent XII (1691–1700). While Innocent XII was indeed bearded (but just barely—looks more like a glorified soul patch to me), the website is a humour site, so I don’t have full confidence in its accuracy.
(As for the first bearded pope, in case anyone is wondering, it’s fairly certain that the first pope, Peter, was bearded. He was a Jew, and Jewish custom at the time was for men to have beards.)
I can believe that. It’s been the general rule in the Roman rite for the past thousand years or so for clergy to be clean-shaven (with the exception of a few religious orders). Enforcement of this has waxed and waned, and obviously if a pope wants to have facial hair, he can have it, but the past few centuries were rather strict about it. After Vatican II, facial hair is now optional, not prohibited (though still the norm).
If you look at the popes before Innocent XII in that Wiki article, you will note that Alexander VIII, Clement X, Clement IX, and Alexander VII are all depicted with similar styles of facial hair (they don’t have a portrait of Innocent XI). If there was a general rule, the 17th century popes don’t appear to have felt bound by it. Nor did Cardinal Richelieu, who also affected that style. It was a fairly common style for 17th century aristocrats (Charles I, for example), and appears to have been popular with high ranking Catholic clergy as well.
Indeed, as the Eastern rites are to this day. As in so many things, the Catholic Encyclopedia gives a good potted history of this peculiarity of the Roman rite.
The article yBeayf mentions claims that beards have been prohibited since about 503, but doesn’t give any reason for the injunction. However, it does quote the rationale given by the 13-Century Church scholar Guillaume Durand:
Later Renaissance-era writings give other reasons: