Regulus bulgaricus is a fossil passerine from the Middle Villafranchian (upper Pliocene to lower Pleistocene ) of Bulgaria.

The only known specimen of Regulus bulgaricus is a complete left ulna, 13.3 mm in length.

It is distinguished from R. regulus by a thicker base, a longer olecranon, a larger cotyla dorsalis, and smaller quill knobs (papillae remigales caudales).

Compared to R. ignicapilla, R. bulgaricus has a narrower proximal part of the diaphysis, a shorter olecranon, and smaller tuberculum retinaculi.

Although Boev was unable to compare the fossil ulna with R. goodfellowi, it can be excluded from this taxonomical comparison due to it sharing a superspecies with R.