Background: Staphylococcus aureus is a major human pathogen responsible for a large number of infections. In S. aureus, SarA is an important global locus responsible for the regulation of virulence factors, as well as biofilm formation. Objectives: The aim of this work is to clarify the impact of SarA on biofilm formation, immune system evasion, as well as the survival of S. aureus under stress conditions. Methodology: A comparative study between S. aureus wild type strain, sarA mutant and complemented strains was established addressing the biofilm formation, opsonization, phagocytosis, as well as ability of the bacterium to survive in stressful environments including acidic pH, hyperosmotic and oxidative stress. The in vitro experiments were confirmed by challenging of mice via intraperitoneal injection with the wild type strain, sarA mutant and complemented strains. Results: Mutation of sarA diminished significantly biofilm formation. Moreover, this mutation resulted in a slight decrease in the deposition of the most important opsonin in complement-mediated immunity, named C3 on S. aureus cells. However, this mutation was associated with a significant enhancement of bacterial phagocytosis and killing by human neutrophils. Furthermore, this mutation altered bacterial survival in stressful conditions. It is also noteworthy that sarA mutation resulted in a significant higher survival rates during the challenging of mice. Conclusion: SarA plays a role as a key regulator of biofilm formation, which in turn has a great impact on immune system evasion through affecting opsonization and phagocytosis. In addition, SarA improves the ability of S. aureus to survive in stressful conditions.