Semiconducting photocatalytic solar–hydrogen conversion (SHC) from water is a great challenge for renewable fuel production. Organic semiconductors hold great promise for SHC in an economical and environmentally benign manner. However, organic semiconductors available for SHC are scarce and less efficient than most inorganic ones, largely due to their intrinsic Frenkel excitons with high binding energy. In this study the authors report polymer heterojunction (PHJ) photocatalysts consisting of polyfluorene family polymers and graphitic carbon nitride (g‐C3N4) for efficient SHC. A molecular design strategy is executed to further promote the exciton dissociation or light harvesting ability of these PHJs via alternative approaches. It is revealed that copolymerizing electron‐donating carbazole unit into the poly(9,9‐dioctylfluorene) backbone promotes exciton dissociation within the poly(N‐decanyl‐2,7‐carbazole‐alt‐9,9‐dioctylfluorene) (PCzF)/g‐C3N4 PHJ, achieving an enhanced apparent quantum yield (AQY) of 27% at 440 nm over PCzF/g‐C3N4. Alternatively, copolymerizing electron‐accepting benzothiadiazole unit extended the visible light response of the obtained poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole)/g‐C3N4 PHJ, leading to an AQY of 13% at 500 nm. The present study highlights that constructing PHJs and adapting a rational molecular design of PHJs are effective strategies to exploit more of the potential of organic semiconductors for efficient solar energy conversion.