The development of efficient heterogeneous catalysts for degrading organic pollutants or converting them into harmless and even useful products is of vital significance for environmental remediation. Herein, we reported the simple synthesis of highly active and stable graphitic carbon nitride (g-C3N4) nanosheet–supported ZnS composites (g-C3N4/ZnS) and demonstrated its excellent performances as a catalyst for the reductive conversion of 4-nitrophenol (4-NP). The prepared g-C3N4/ZnS composites were characterized by a collection of analytical techniques. The catalytic activity exhibited by the g-C3N4/ZnS composite was 4.1 and 1.9 times higher than that of g-C3N4 and ZnS, respectively. The remarkably increased catalytic performance of the g-C3N4/ZnS composite was mainly attributed to the synergistic effect of the junction or interface formed between g-C3N4 and ZnS, resulting in enhanced catalytic activity. Notably, the synthesized g-C3N4/ZnS composite was found to exhibit an outstanding catalytic activity and excellent recyclability for five cycles in the reduction of 4-NP using NaBH4 without considerable loss of catalytic activity, demonstrating its long-term stability. This work shows significant progress for low-cost fabrication of the g-C3N4/ZnS composite with excellent catalytic activity.