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Heterojunction perovskite solar cells combine the stability of 2D perovskites and the high efficiency of 3D perovskites, making them an excellent photovoltaic candidate. While heterojunctions with intermixed or gradient perovskites can reduce surface recombination, the aggregation and phase distribution of 2D perovskite induce transport losses, thereby limiting device fill factors. Accordingly, a bulk in situ reconstruction (BISR) strategy is proposed to induce the reconstruction of 3D perovskites on a minim self‐assembled 2D crystal seed, forming heterojunction perovskite that runs through the entire active layer. This facilitates charge extraction, relieves tensile stress, and avoids the decomposition of perovskite on grain boundaries. As a result, the best‐performing heterojunction perovskite solar cells show a high‐power conversion efficiency (PCE) of 24.06% with 82.9% FF for the small‐area device (0.105 cm2) and a superior PCE of 19.2% for the large‐area module (5 × 5 cm2). Importantly, the unencapsulated device shows dramatically improved operational stability, maintaining 87% of its initial efficiency after 8000 h of storage under ambient atmosphere at room temperature. This work provides an effective and simple approach to establish heterojunction perovskite to simultaneously boost the efficiency and stability of PSCs. [ABSTRACT FROM AUTHOR]