Tembusu virus (TMUV) is an emerging mosquito-borne flavivirus, primarily transmitted by spp. mosquitoes. The 2010 outbreak of TMUV in ducks revealed that the virus had acquired direct contact and aerosol transmission routes, enabling its rapid spread in duck farms. Recently, cluster 3 TMUV has increasingly been isolated from chickens, ducks and geese. In this study, we examined the pathogenicity and transmission routes of the cluster 3 TMUV Shandong 2021 (SD) strain in ducks and chickens. Our results show that TMUV SD can infect both species, but only in ducks could TMUV be detected in throat and cloacal swabs. In ducks, the virus can spread without mosquito involvement to co-housed naïve birds, demonstrating direct transmission capability. Conversely, no virus shedding and direct transmission were observed in chickens, suggesting that mosquitoes are required for virus transmission between chickens. Indeed, mosquitoes could become infected by biting chickens infected with the TMUV SD and transmit the virus to naïve chickens. Our results, for the first time, provide direct evidence that TMUV can be transmitted by mosquitoes in a laboratory setting. Furthermore, our findings indicate that viral excretion through the respiratory tract and/or digestive tract is essential for direct contact transmission of TMUV, which is a critical factor in the epidemic spread of TMUV. These insights into the transmission dynamics of a cluster 3 TMUV emphasize the importance of effective vector control and biosecurity measures in managing and preventing outbreaks.