Background: In south-eastern Tanzania where insecticide-treated nets have been widely used for more than 20 years, malaria transmission has greatly reduced but remains highly heterogenous over small distances. This study investigated the seasonal prevalence of Plasmodium sporozoite infections in the two main malaria vector species, Anopheles funestus and Anopheles arabiensis for 34 months, starting January 2018 to November 2020. Methods: Adult mosquitoes were collected using CDC-light traps and Prokopack aspirators inside local houses in Igumbiro and Sululu villages, where earlier surveys had found very high densities of An. funestus. Collected females were sorted by taxa, and the samples examined using ELISA assays for detecting Plasmodium falciparum circumsporozoite protein (Pf-CSP). Results: Of 7,859 An. funestus tested, 4.6% (n = 365) were positive for Pf sporozoites in the salivary glands. On the contrary, only 0.4% (n = 9) of the 2,382 An. arabiensis tested were positive. The sporozoite prevalence did not vary significantly between the villages (p = 0.36) or seasons (p = 0.59). Similarly, the proportions of parous females of either species were not significantly different between the two villages (p > 0.05) but was slightly higher in An. funestus (0.50) than in An. arabiensis (0.42). Analysis of the 2020 data determined that An. funestus contributed 98% of all malaria transmitted in households in these two villages. Conclusions: Despite the widespread use and overall impact of ITNs, there is still excessively high Plasmodium infection prevalence in the dominant malaria vector, An. funestus, causing intense year-round malaria transmission in the study villages. Further reduction in malaria burden thus requires effective targeting of An. funestus in these and other villages with similar epidemiological conditions.