The halogen end group of polymers prepared via atom transfer radical polymerization (ATRP) can be converted into other functional groups by different chemical modifications. Herein, a new method to modify the bromine end group into a thiol functionality in (meth) acrylic polymers synthesized by ATRP is reported for the first time. Thus, the bromine end group of several poly(methyl methacrylate)s was successfully converted into a thioacetate and then in a mercapto group by a controlled hydrolysis. This end-functionalization was confirmed introducing in situ a fluorescent group by thiol-ene "click" reaction with a synthetic alquene tethered to a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) fragment. In addition, two model reactions in which bromide ATRP initiators were transformed into new thiol-functionalized molecules proved that this methodology can be applied to either methacrylic or acrylic polymers synthesized by ATRP.