AbstractTwo‐dimensional (2D) conjugated polymers and their layer‐stacked 2D conjugated covalent organic frameworks, such as 2D poly(arylene vinylene)s (2D PAVs), are emerging as promising polymer semiconductors for electronics and photocatalysis. However, achieving narrow optical band gaps and efficient electron transport remains a significant challenge for this class of materials to enhance the device's performance. Here, we report a donor‐acceptor‐type 2D PAV (2DPAV‐TBDT‐IT, where TBDT = thienyl‐benzodithiophene and IT = s‐indacene‐1,3,5,7(2H,6H)‐tetraone) synthesized via an Aldol‐type 2D polycondensation approach. Notably, 2DPAV‐TBDT‐IT benefits from an effective intralayer donor–acceptor effect, exhibiting an optical band gap of 1.15 eV, the smallest among the reported 2D conjugated polymers. Density functional theory calculations reveal a unique electron‐dominating transport for 2DPAV‐TBDT‐IT, with a strongly dispersive conduction band minimum and, thus, a small effective mass for electrons half that for holes. Additionally, terahertz spectroscopy measurements indicate a high charge mobility of 26 cm2 V−1 s−1 at room temperature for the powder sample. Given the high electron‐deficiency of 2DPAV‐TBDT‐IT for facile electron injection from hazardous gases and the high‐mobility electron‐dominating transport in the material, we further fabricate chemiresistors from 2DPAV‐TBDT‐IT, showing ultrasensitive SO2 analyte detection with limit of detection of 0.088 ppb, significantly surpassing the reported chemiresistive SO2 sensors.