Background <p>Gastric cancer (GC) is a leading contributor to cancer-related illness and death globally, presenting a significant public health concern in Iran. There is a lack of extensive data regarding long-term trends at both national and regional levels for GC incidence and its anticipated future impact. This research aimed to investigate the changes in gastric cancer rates in Iran from 1990 to 2021 and to forecast incidence rates through 2030 by gender, age groups, and provinces.</p> Methods <p>This research used secondary ecological data from the GBD 2021 research to investigate the incidence of GC in Iran from 1990 to 2021. Temporal trends were assessed with Joinpoint Regression to determine the Average Annual Percent Change (AAPC). Also, a Bayesian age-period-cohort model was then applied to project new cases and incidence rates up to 2030. We used from R and Joinpoint software for all analyses.</p> Results <p>From 1990 to 2021, Iran saw nearly a twofold increase in new GC instances; however, the age-standardized incidence rate (ASIR) significantly fell from 23.12 to 14.37 per 100,000 individuals. The most significant decreases were observed in the northwestern provinces and among females. Forecasts for 2030 show that the absolute number of cases will likely increase to around 12,070, yet the ASIR is predicted to see a slight drop to 13.44, with a stagnation in decrease for individuals over 80 years old, suggesting a gradual slowdown in the overall trend.</p> Conclusions <p>Although there has been a persistent reduction in the age-standardized incidence rate of GC in Iran over the last thirty years, the sheer number of cases is still increasing owing to demographic growth and an aging population. To combat this rising challenge, future initiatives should focus on implementing targeted screening in high-risk areas and for elderly populations, improving geriatric healthcare, and addressing inequalities in healthcare access through better collaboration across sectors.</p>