Obesity is associated with co-morbidities and increased risk of acquiring infections with worse outcomes. Paradoxically, a few studies indicate that obesity may have a decreased mortality in hospitalized patients with pneumonia. The objective of this study was to determine the impact of body mass index (BMI) on short-term all-cause mortality and clinical outcomes among hospitalized adults with pneumonia, urinary tract infections, skin and soft tissue infections, and bacteremia. The study cohort included 1437 consecutive patients who were admitted with infectious disease including pneumonia (717), urinary tract infection (506), bacteremia (69), and skin and soft tissue infections (145), and hospitalized in internal medical departments, during 2013-2015. BMI was categorized as underweight (≤20 kg/m2), normal (20-25 kg/m2), overweight (25.1-29.9 kg/m2), and obese (≥30 kg/m2). Clinical outcomes of 30- and 90-day all-cause mortality rates, length of hospital stay, and transfer to the intensive care unit (ICU) were compared among groups, sorted according to BMI and different infectious diseases. Obesity was associated with decreased 30-day mortality in patients with pneumonia [odds ratio (OR) = 0.26, 95 % confidence interval (CI) 0.06-1.01; p = 0.052]. On the contrary, increased 30-day mortality was observed in the underweight patients (OR = 2.89, 95 % CI 1.1-7.6; p = 0.03). Similar impacts were not found for urinary tract infections, skin and soft tissue infections, or bloodstream infections. Furthermore, obesity had no effect on 90-day mortality, length of hospital stay, or transfer to the ICU in all kinds of infectious diseases. Obesity is associated with reduced short-term mortality among hospitalized patients with pneumonia. Whether gut microbiota in obese individuals plays a role in this protective effect remains to be investigated by further studies.