Late blight, caused by Phytophthora infestans, is a devastating disease of tomato, primarily driven by asexual reproduction through sporangia and zoospores. This study presents the first confirmed report of self-fertile P. infestans isolates in Northeast India, capable of producing sexual oospores, signifying a major shift in the pathogen's survival strategy. Infected tomato plants collected between November and December were cultured on rye agar at 18 ± 1°C. Oospore initiation occurred after 28 days, with mature oospores averaging 35 µm in diameter. These self-fertile isolates formed oospores independently and exhibited pathogenicity as confirmed by Koch's postulates. Field observations over five years in semi-temperate conditions (10–17°C, high humidity) confirmed oospore formation in senescing tissues. This discovery suggests that P. infestans may now overwinter in soil via oospores, increasing its persistence and genetic diversity, and possibly resulting in earlier and more severe outbreaks. These findings highlight the urgent need for integrated management strategies targeting both asexual and sexual propagules.