SARS-CoV-2 infection usually results in multi-organ affection, and severe inflammatory response which associated with significant affection of blood cell count and coagulation profile. We aimed to study the changes in haematological parameters among patients infected with SARS-CoV2 and its relation to the severity of infection. A retrospective cohort investigated data from patients hospitalized at the Armed Forces Hospital in Jeddah, Saudi Arabia, between July 2020 and July 2022. Patient who tested positive for SARS-CoV 2 infection were compared to negative patients. A total of 586 patients were included, 387 tested positive for SARS-CoV-2. COVID-19 patients had significant lower leukocyte, and eosinophil counts along with lower red blood cell indices, suggesting a potential impact on the hematopoietic system. Deviation in leukocyte count was correlated with disease severity. the mortality was higher in the non-COVID-19 group (14.1 vs. 7.5%). COVID-19 patients had significantly higher fibrinogen levels and lower prothrombin time than non-COVID-19 patients. No difference was found regarding liver enzymes, renal function tests or acute-phase reactants. Haematological parameters could be used as markers for early detection, assessment of disease severity, and prognosis of SARS-CoV-2 infection. Further studies are necessary to understand the underlying mechanisms and their clinical significance.