Andrallus spinidens Fabricius (Hemiptera: Pentatomidae) is an important biological control agent of lepidopteran larvae, including the rice green caterpillar, Naranga aenescens Moore (Lepidoptera: Noctuidae), in rice fields of northern Iran. Considering the importance of conserving natural enemies to reduce pesticide use and its adverse effects, this study aimed to (1) compare the population densities of A. spinidens and N. aenescens in rice fields across seven counties of western Mazandaran Province (Mahmoudabad, Noor, Nowshahr, Chalous, Abbasabad, Tonekabon, and Ramsar), and (2) investigate their seasonal population dynamics in two unsprayed rice fields located in Tonekabon and Nowshahr. Field surveys were conducted from 2014 to 2016 using sweep-net sampling during the growing season and direct observations after harvest. A. spinidens was recorded in all surveyed fields, even where the pest was absent. The highest predator densities were recorded in Abbasabad (41 individuals) in 2014 and Tonekabon (14 individuals) in 2015, both in fields without insecticide application. In contrast, the highest pest densities were observed in Ramsar (51 individuals) in 2014 and Nowshahr (105 individuals) in 2015. Seasonal monitoring showed that the pest population peaked in July (111 individuals) and August (62 individuals), while the predator also reached its highest densities during July (39 individuals) and August (21 individuals). Overall, the synchronized seasonal fluctuations of A. spinidens and N. aenescens, particularly in unsprayed fields, highlight the predator's potential contribution to integrated pest management programs in northern Iranian rice ecosystems.