Near-infrared (NIR) organic phototransistors (OPTs) based on diketopyrrolopyrrole (DPP) conjugated polymers with partially removed long branched alkyl side chains are reported with significantly improved NIR photoresponses, which take advantage of both strengthened NIR absorption and improved charge transport. The NIR OPTs based on the terpolymer with properly partial removal of the alkyl side chains showed much higher photoresponsivity R (567 A W⁻¹) and larger photosensitivity P (∼10⁶) as compared to those of the other two control devices based on DPP conjugated polymers without removal or with excessive removal of the alkyl side chains. By incorporating a small amount of [6,6]phenyl-C61-butyric acid methyl ester into the terpolymers with partially removed side chains, the resulting NIR OPTs exhibited ultralow detection limit (0.002 mW cm⁻²) and presented very high sensitivity with a maximum R of 4234 A W⁻¹ and specific photodetectivity D* of 5.6 × 10¹³ Jones. These findings demonstrated that side chain engineering in conjugated polymers provides an excellent opportunity for developing high performance organic photodetectors.