Fluorescence imaging at longer wavelengths,especially in the shortwave-infrared (SWIR: 1000−1700 nm)region, leads to a substantial decrease in light attenuation,scattering, and background autofluorescence, thereby enablingenhanced penetration into biological tissues. The limited selectionoffluorescent probes is a major bottleneck in SWIRfluorescenceimaging. Here, we develop SWIR-emitting nanoparticles composedof donor−acceptor-type conjugated polymers. The bright SWIRfluorescence of the polymer dots (primarily attributable to theirlarge absorption cross-section and highfluorescence saturationintensity (as high as 113 kW·cm−2)) enables the unprecedenteddetection of single particles as small as 14 nm through millimeter-thick turbid media. Unlike most SWIR-emitting nanomaterials,which have an excited-state lifetime in the range of microseconds to milliseconds, our polymer dots exhibit a subnanosecond excited-state lifetime. These characteristics enable us to demonstrate new time-gated single-particle imaging with a high signal-to-background ratio. Thesefindings expand the range of potential applications of single-particle deep-tissue imaging.