Biostimulants such as humic acid (HA), salicylic acid (SA), and ascorbic acid (ASA) have gained increasing attention for enhancing crop resilience and productivity. However, their effectiveness is strongly influenced by environmental and nutritional contexts. A field experiment was conducted in two contrasting climates of Iran—semi-humid (Khorramabad) and arid (Mashhad)—to evaluate the combined effects of nitrogen sources and foliar biostimulants on physiological, biochemical, and yield traits of cluster bean (Cyamopsis tetragonoloba L.). The experiment was conducted as a factorial arrangement within a randomized complete block design with three replications. The first factor included five nitrogen treatments: control (no N), urea (69 kg N ha⁻¹), ammonium nitrate (115 kg N ha⁻¹), vermicompost (8 t ha⁻¹; equivalent to 120 kg N ha⁻¹), and biological N fertilizer containing Azotobacter spp. (Azetobarvar). The second factor comprised four foliar spray treatments: distilled water (control), HA (0.2 g L⁻¹), ASA (200 mg L⁻¹), and SA (200 µM). Significant three-way interactions among location, fertilizer, and biostimulant were observed for photosynthesis rate, pigments, biochemical compositions, and yield. Urea + HA consistently produced the highest photosynthetic rate and seed yield (up to 3526 kg ha⁻¹), whereas SA enhanced stress tolerance and protein content under arid conditions. Correlation analysis showed that seed yield was positively associated with photosynthesis rate (r = 0.35^**) and biological yield (r = 0.89^**). Overall, HA maximized nutrient use efficiency in favorable environments, while SA and ASA enhanced biochemical performance. These findings underscore the potential of integrated nitrogen–biostimulant management for sustainable legume production across diverse climates.