Multifunctional theranostics play a critical role in improving the efficacy of photothermal therapy and tumor fluorescence imaging; however, they require the integration of complex components into a single theranostic system, and their response in the second near-infrared (NIR-II) region is constrained by wavelengths of a photosensitizer. To address this issue, we herein developed a novel multifunctional thiazole-fused quinoxalineimide semiconducting polymer (named PQIA-BDTT), which exhibits NIR-II fluorescence and photothermal properties. PQIA-BDTT nanoparticles achieved an impressively high photothermal conversion efficiency (72.6%) in laser (1064 nm)-induced photothermal therapy at a safe maximum permissible exposure, demonstrating their capability as an effective photothermal agent. Moreover, PQIA-BDTT nanoparticles can be used as a reference for NIR-II fluorescence imaging under a low laser fluence. The tumor size and location in 4T1 mice intravenously injected with the PQIA-BDTT nanoparticles could be precisely identified through NIR-II fluorescence imaging, which also exhibited remarkable photothermal antitumor efficacy by and therapy. Overall, this study demonstrates that introducing a thiazole-fused quinoxalineimide acceptor unit into a donor-acceptor conjugated polymer is an effective strategy for the synthesis of novel multifunctional theranostic systems, which provides a novel platform for designing theranostic agents for biomedical applications.