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Abstract Molecular diodes are of considerable interest for the increasing technical demands of device miniaturization. However, the molecular diode performance remains contact‐limited, which represents a major challenge for the advancement of rectification ratio and conductance. Here, it is demonstrated that high‐quality ultrathin organic semiconductors can be grown on several classes of metal substrates via solution‐shearing epitaxy, with a well‐controlled number of layers and monolayer single crystal over 1 mm. The crystals are atomically smooth and pinhole‐free, providing a native interface for high‐performance monolayer molecular diodes. As a result, the monolayer molecular diodes show record‐high rectification ratio up to 5 × 108, ideality factor close to unity, aggressive unit conductance over 103 S cm−2, ultrahigh breakdown electric field, excellent electrical stability, and well‐defined contact interface. Large‐area monolayer molecular diode arrays with 100% yield and excellent uniformity in the diode metrics are further fabricated. These results suggest that monolayer molecular crystals have great potential to build reliable, high‐performance molecular diodes and deeply understand their intrinsic electronic behavior.