A modular lighting device includes a base and a lamp head unit. The base includes a base body and a first connecting module disposed at a top end of the base body and having a first connecting portion and a first conductive portion. The lamp head unit includes a bottom seat, a lamp body disposed on the bottom seat, and a lamp shell connected to the bottom seat. The bottom seat includes a second connecting module disposed at a bottom end thereof and having a second connecting portion connected removably to the first connecting portion, and a second conductive portion electrically connected to the first conductive portion. The lamp shell has a maximum width disposed at a position adjacent to a bottom end thereof and smaller than or equal to that of the bottom seat.