In Caspian languages such as Gilaki and Tatic ones like Tati and Talyshi, the formation of passive voice involves the use of periphrastic verbs like bubon/bostən and vəkətən “become,” along with the suffix -i- or -ə- for the Tatic group. The former, bubon/bostən, can be traced back to Middle Iranian (MIr.) *bauH, while the latter derives from MIr. -īy- < -ya/-iia. Previous scholarship described periphrastic passive constructions in Gilaki and both periphrastic and morphological passive constructions in Tatic. However, this study reveals the existence of a morphological passive in a dialect of Gilaki, Foomanati. The research demonstrates that in Gilaki Foomanati, the passive voice can also be formed morphologically by appending the suffix -əx-~-x-. The plausibility of tracing -əx- back to the MIr passive suffix -īh- will be thoroughly examined. Additionally, the study investigates whether -əx- is developed due to language contact with other languages like Tatic or if it is an inherited feature specific to Gilaki dating back to its earlier linguistic period. The data presented is substantiated by evidence of the passive suffix -əx- in other Gilaki dialects, corroborating the hypothesis of a h → x sound change observed in Gilaki and Gilaki Foomanati. Through meticulous analysis and comparison, this study aims to deepen our understanding of passive voice formation in Caspian languages and shed light on the historical development of linguistic features within the region.