There have been a few studies on the seasonal variation of bloodstream infections or microorganisms isolated from blood cultures. The aim of this study was to clarify the seasonal variation of microorganisms isolated from blood cultures and their relationship with the climate. A total of 30,945 blood cultures over 10 years between January 2005 and December 2014 were included in this study. Blood culture positivity rate (BCPR) was defined as the number of infection- or microorganism-positive blood cultures per 1,000 person-days of hospital admissions, which were analyzed monthly or seasonally. The BCPR was significantly higher in the summer than in the spring, and positively correlated with temperature or humidity, primarily owing to the seasonal variation of gram-negative rods (GNRs) and Bacillus spp. The BCPRs of coagulase-negative staphylococci, Bacillus spp., and Enterococcus spp. positively correlated with temperature, while the BCPRs of GNRs and Candida spp. positively correlated with humidity; however, the BCPR of Staphylococcus aureus negatively correlated with humidity. Overall, the BCPR showed seasonal variations and was associated with temperature and humidity depending on the bacterial strain.