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Arsenic-containing conjugated polymers have recently attracted considerable attention, and dithieno[3,2-b:2′,3′-d]arsole is a typical monomer unit often used to construct them. In conventional polymer design, long alkyl chains, which improve the solubility of the resultant polymers in organic solvents, should be introduced on the coupling partner and/or at the 3-positions of the fused thiophene rings. The former strategy cannot be used for comonomers in which alkyl group substitution is difficult, and in the latter strategy, the steric hindrance of the alkyl chains may distort the planarity of the polymer chain. Therefore, we developed a novel monomer in which an alkoxyphenyl group is substituted on the arsenic atom. Polymers with high planarity and solubility were developed using benzothiadiazole and benzoxadiazole monomers. Most importantly, the polymers exhibited near-infrared emissions (emission maximum: up to 886 nm) owing to their planarity.