Tin oxide has been the mainstream as the electron transport layer for highly efficient perovskite solar cells due to advantages such as high mobility, high transmittance, and strong UV stability. Realizing high‐quality SnO₂ layer at low temperature can widen the range of potential applications on both rigid and flexible substrates. Herein, a seed‐assisted growth strategy is developed for the preparation of SnO₂ layer at low temperature (100 °C) and the seeds in precursor solution provide preferential nucleation sites. The SnO₂ layer via seed‐assisted growth exhibits higher conductivity and mobility which are more than twice those of control samples because of enhanced crystallinity. The resulting solar cells show high V_oc of 1.189 V, contributing to a high power conversion efficiency of 25.26%. To current knowledge, this is the highest efficiency for perovskite solar cells with SnO₂ layer formed at low temperature (≤100 °C). Contributed by the high‐quality SnO₂ layer prepared at low temperature, a high power conversion efficiency over 22% for flexible perovskite solar cells is also achieved.