I've seen it referred to one from time to time, but it doesn't fit the accepted definition of one. Because it fires the 5.45x39mm rifle round and not a handgun round, it's just a short carbine.
(Edit: See also the closely-related PP-19-01, which, firing 9x19mm, is an actual submachine gun.)
Well, it uses 5,45x39mm rounds but it's as compact as an SMG. And after reading a bit I found that the AKS-74U can actually be called the bridge between SMG and assault rifle.