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Herein, ultralong α‐CaSO4·0.5H2O whiskers with an average length above 500 µm and aspect ratios of 200–500 are hydrothermally produced employing trace amount (0.06 mg L−1) of cationic polyacrylamide (CPAM) as the morphology modifier. The hydrothermal formation mechanisms of α‐CaSO4·0.5H2O involving the initial nucleation of bassanite nanocrystals/nanorods and their subsequent self‐assembly into long whiskers through oriented attachment are revealed for the first time. Experimental results indicate that CPAM is electrostatically adsorbed onto the initial bassanite nanorods and results in their near‐neutral surfaces, favoring the assembly of the initial nanorods into larger aggregates, thus finally leading to the formation of ultralong α‐CaSO4·0.5H2O whiskers with high aspect ratios. Moreover, α‐CaSO4·0.5H2O whiskers are calcined to CaSO4 whiskers for the fabrication of polyvinyl chloride (PVC)/CaSO4 composites with a high impact strength and flexural strength of 42.0 KJ m−2 and 53.6 MPa, respectively, indicating that the mechanical properties of the PVC/CaSO4 composites could be significantly improved by these ultralong CaSO4 whiskers.