Insect-specific flaviviruses (ISFs) are a group of flaviviruses that can replicate efficiently in arthropods but are unable to replicate in vertebrate hosts. This contrasts with medically important flaviviruses, such as dengue (DENV), Zika (ZIKV) and West Nile (WNV) viruses. As ISFs do not cause disease in humans, they are not a human health concern. Importantly, ISFs have uncovered new insights into flavivirus evolution, virus-host transmission mechanisms and various biotechnological applications. This has resulted in an exponential increase in ISF discovery in the past 15 years. Using the established MAVRIC assay (monoclonal antibody against viral RNA intermediates in cells), we report the first isolation of an ISF in Singapore from three pools of Culex gelidus mosquitoes that showed a partial NS5 sequence homology of 95% to Quang Binh virus (QBV). QBV is an ISF first isolated from Vietnam in 2009 and has since been detected recurrently throughout China. To determine the spatiotemporal prevalence of QBV, a total of 17,070 mosquitoes were screened using a QBV-specific RT-qPCR assay, revealing 36 QBV-positive pools of mosquitoes detected mainly along northern coastal regions of Singapore. Repeated detections over 12-months in a north-western nature reserve suggests local stable establishment of the virus. Phylogenetic and molecular analyses show that QBV sequences from Singapore group together with other Southeast Asian sequences, and that Culex gelidus-derived sequences are phylogenetically distinct from those derived from Culex tritaeniorhynchus. Like other ISFs, these QBV isolates grew efficiently in C6/36 mosquito cells but failed to replicate in a Vero vertebrate cell line. We also show that QBV was able to downregulate DENV and WNVKUN in C6/36 cells by 2.9 logs and 1.8 logs respectively. This report represents the first known spatiotemporal study of an ISF and highlights QBV’s potential as a biological control against medically important flaviviruses.