Legionellosis, an infection caused by the environmental bacteria Legionella spp., has become a significant public health problem in the United States in recent years; however, among the states the incidence rates vary widely without clear explanation. This study examined environmental effects on the 2014-2016 average annual legionellosis incidence rates in the US states through correlative analyses with long-term precipitation, temperature, solar ultraviolet radiation, and sunshine hours. The continental states west of approximately 95 degrees W showed low incidence rates of 0.51 to 1.20 cases per 100,000 population, which corresponded to low precipitation below annual 750 mm. For the eastern states, where precipitation was higher, solar effects were prominent and mixed, leading to wide incidence variation. Robust regressions suggested a dividing line at 40 degrees N: north of this line, rising temperature, mainly from solar heat, raised legionellosis incidence, to the peak of 4.25/100,000 in Ohio; south of this line, intensifying sunlight in terms of high ultraviolet indices and long sunshine hours prevailed to limit incidence, gradually to 0.99/100,000 in Louisiana. On or near the 40 degrees N line were 15 eastern states that had leading legionellosis incidence rates of >2.0/100,000. These states all showed modest environmental parameters. By contrast, the frigid climate in Alaska and the strong year round solar ultraviolet in Hawaii explained the least incidence of 0.14/100,000 and 0.47/100,000 in these states respectively. The findings of solar and climate effects explain the wide variation of legionellosis incidence rates in the US and may offer insights on the potential exposure and prevention of the infection.Importance Legionellosis, caused by environmental bacteria Legionella spp., has become a significant public health problem in the United States in recent years with approximately 6000 cases annually. The present study showed, through a series of correlative analyses with long-term precipitation, temperature, solar ultraviolet radiation, and sunshine hours, that these environmental conditions strongly influence the legionellosis incidence rates across the US in mixed and dynamic fashions. The incidence rates varied remarkably by regions, with the highest in Ohio and New York and the lowest in Alaska. A precipitation threshold above 750 mm was required for elevated legionellosis activity. Regression models and dividing lines between regions were established to show the promotive effect of temperature as well as the inhibitive effects of solar ultraviolet and sunshine hours. These findings explain the wide variation of legionellosis incidence rates in the US. They may also offer insights on the potential exposure and prevention of the infection.