Flowerlike MoS2 microspheres were synthesized through a hydrothermal method. 2H-MoS2 nanoparticles, MoS2/MoO3 heterojunctions, and α-MoO3 nanoplates were prepared by annealing the MoS2 microspheres under different reaction conditions. The formation and growth mechanism of the samples from flowerlike MoS2 microspheres to α-MoO3 nanoplates is explained in detail. The photocatalytic properties of the four samples for the degradation of rhodamine B (RhB) under visible-light irradiation were studied. The results showed that the flowerlike MoS2 microspheres, MoS2/α-MoO3 heterojunctions, and α-MoO3 nanoplates all have excellent photocatalytic activities. In particular, the flowerlike MoS2 microspheres exhibit the highest photocatalytic activity for the degradation of RhB.