The experiment was conducted to investigate the potential effects associated with dietary guanidinoacetic acid (GAA) supplementation and different stocking densities on behavioral patterns and welfare indicators of broiler chickens. In total, 364 Arbor-Acres one-day-old mixed-sex broiler chicks were randomly assigned to four treatments of 2 × 2 factorial arrangement in a completely randomized design experiment (seven replicates each). The first treatment (T1): standard stocking density (SSD) reared under 10 birds/m² without GAA supplements, treatment 2 (T2): SSD (10 birds /m²) with GAA supplements at the rate of 0.06% (600 g/ton feed), treatment 3 (T3): high stocking density (HSD) reared under 16 birds/m² without GAA supplements, treatment 4 (T4): HSD (16 birds/m²) with 0.06 % GAA supplements. Scan sampling behavior observations were recorded through video recording cameras and welfare indices were measured using 3 levels scale score. Results of the current study showed a positive increase in foraging and comfort behavior in SSD, while an increase in standing and a decrease in sitting was observed in HSD. Furthermore, GAA resulted in raising foraging behavior in SSD, however, no effect was noted on standing, resting, and comfort behavior. Based on interaction analysis, a higher standing behavior was observed by GAA supplements in the HSD group. Regarding the footpad dermatitis (FPD) and hock burns score, HSD was reported to negatively affect leg health as it significantly increases FPD and hock burns score, however, neither GAA nor interaction between Stocking density x GAA had a significant effect on welfare indices. In conclusion, HSD had a negative impact on the behavior and welfare of broiler chickens as it showed adverse effects on leg health. However, the positive effects of GAA on some behavioral patterns were more pronounced in the SSD group than in the overcrowding conditions.