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Rift Valley fever (RVF) is a mosquito-borne viral zoonotic disease that causes significant economic and public health impacts in Kenya. Currently, there is inadequate information on rainfall patterns and mosquito diversity, including the dominant species in Nyandarua. Therefore, this study aimed to determine the rainfall patterns, relative abundance, and diversity of mosquito species that can transmit the RVF virus (RVFV) in Nyandarua. Mosquitoes were purposively collected from all the homesteads with suspect RVF cases in 10 villages using the CDC light trap during the dry (January 2021), long (July 2020), and short (November 2021) rainy seasons. Rainfall data was derived from the Climate-SERV satellite database. The mosquitoes were identified morphologically using a dissecting microscope and their identities, as well as genetic diversity, were determined using sequencing and phylogenetic analysis of the CO1 gene. A total of 97, 328, and 366 mosquitoes were trapped during the dry, long and short rainy seasons, respectively. There was variation in the average daily rainfall between 2015 and 2021 during the three seasons. Of the mosquitoes trapped, 71 (9%) were males while 720 (91%) were females with 26 (4%) mosquitoes being blood-fed. During the three seasons, various species of mosquitoes including Culex pipiens (58%), Culex theileri (16%), Culex vansomereni (3%), Culex rima (2%), and Culex perexiguus (3%) were identified (Simpson index = 0.4). Culex pipiens was the most dominant species in this ecosystem (Shannon index = 1.2). We conclude that the anomalous variations in rainfall patterns may be correlated with the emergence of diverse species of mosquitoes that have the potential to transmit RVFV to animals in Nyandarua.