Bright long-wavelength-excitable semiconducting polymer dots (LWE-Pdots) are highly desirable for in vivo imaging and multiplexed in vitro bioassays. LWE-Pdots have been obtained by incorporating a near-infrared (NIR) emitter into the backbone of a polymer host to develop a binary donor–acceptor (D–A) system. However, they usually suffer from severe concentration quenching and a trade-off between fluorescence quantum yield (Φ) and absorption cross-section (σ). Herein, we describe a ternary component (D/D–A) strategy to achieve ultrabright, green laser-excitable Pdots with narrow-band NIR emission by introducing a BODIPY-based assistant polymer donor as D. The D/D–A Pdots possess improved Φ and σ compared to corresponding binary D–A Pdots. Their Φ is as high as 40.2 %, one of the most efficient NIR Pdots reported. The D/D–A Pdots show ultrahigh single-particle brightness, 83-fold brighter than Qdot 705 when excited by a 532 nm laser. When injected into mice, higher contrast in vivo tumor imaging was achieved using the ternary Pdots versus the binary D–A Pdots.