Motivated by the Lee-Yang approach to phase transitions, we study the partition function of the Generalized Random Energy Model (GREM) at complex inverse temperature β. We compute the limiting log-partition function and describe the fluctuations of the partition function. For the GREM with d levels, in total, there are 1 2 (d + 1)(d + 2) phases, each of which can symbolically be encoded as, the first d 1 levels (counting from the root of the GREM tree) are in the glassy phase (G), the next d 2 levels are dominated by fluctuations (F), and the last d 3 levels are dominated by the expectation (E). Only the phases of the form G d 1 E d 3 intersect the real β axis. We describe the limiting distribution of the zeros of the partition function in the complex β plane (= Fisher zeros). It turns out that the complex zeros densely touch the positive real axis at d points at which the GREM is known to undergo phase transitions. Our results confirm rigorously and considerably extend the replica-method predictions from the physics literature.