BL Lacertae or BL Lac is a highly variable, extragalactic AGN (Active Galactic Nuclei or Active galaxy) originally thought to be an irregular variable star in our own galaxy and so was given a variable star designation. In the 1970s the "star" was identified with a bright, variable radio source and a faint trace of a galaxy was found.

It is the original member of a type of blazar known as "BL Lacertae objects," also termed simply "BL Lac objects."