Hybrid organic-inorganic lead halide perovskite solar cells have shown a remarkable rise in power conversion efficiency over a short period of time; however, long-term stability remains a key challenge hindering the practical application of these cells. Here, we report an approach to sequentially apply a typical one-step solution formulation—self-seeding growth (SSG)—to realize high-quality perovskite thin films with reduced defect density, fewer apparent grain boundaries, improved charge-carrier transport and lifetime, and enhanced hydrophobicity for enhanced stability. Using FA-MA-Cs-based perovskite, SSG devices showed improved efficiency from 17.76% (control) to 20.30% (SSG), with an unencapsulated device retaining >80% of its initial efficiency over 4,680-h storage in an ambient environment with high relative humidities. The SSG devices also exhibited much improved thermal and operational stabilities. In addition, SSG can be applied to different substrates and perovskite compositions, which makes it a viable method for preparing high-quality perovskite thin films for device applications. Display omitted •Self-seeding growth (SSG) to realize high-quality perovskite films are developed•SSG devices show improved efficiency from 17.76% (control) to 20.30% (SSG)•Unsealed devices show better stability under high humidity, heat, and illumination•SSG approach can be applied to different substrates and perovskite compositions The issue of poor long-term stability against moisture is still a key challenge hindering perovskite solar cells for practical applications. Here, we report an approach to sequentially apply a typical one-step solution formulation—self-seeding growth (SSG)—to realize high-quality perovskite thin films with reduced defect density, fewer apparent grain boundaries, improved charge-carrier transport and lifetime, and enhanced hydrophobicity for enhanced stability. Using FA/MA/Cs-based perovskite, SSG devices showed improved efficiency from 17.76% (control) to 20.30% (SSG), with an unencapsulated device retaining >80% of its initial power conversion efficiency over 4,680-h storage in an ambient environment with high relative humidity. In addition, SSG can be applied to different substrates (e.g., SnO2 versus TiO2; planar versus mesoporous) and perovskite compositions, making it a viable method for preparing high-quality perovskite thin films for device applications. A general approach—self-seeding growth (SSG)—that utilizes a typical one-step perovskite precursor ink formulation to create perovskite active layer is reported and can be applied to different substrates and perovskite compositions for preparing high-quality perovskite thin films. Compared to the standard one-step solution-deposited devices, SSG perovskite thin films exhibit reduced defect density, fewer apparent grain boundaries, and improved charge-carrier transport and lifetime. The SSG devices present much improved performance along with better stability under high humidity, heat, and sun illumination.