High-performance phototheranostics with combined photothermal therapy and photoacoustic imaging have been considered promising approaches for efficient cancer diagnosis and treatment. However, developing phototheranostic materials with efficient photothermal conversion efficiency (PCE), especially over the second near-infrared window (NIR-II, 1000–1700 nm), remains challenging. Herein, we report an ultraefficient NIR-II-activated nanomedicine with phototheranostic and vaccination capability for highly efficient in vivo tumor elimination and metastasis inhibition. The NIR-II nanomedicine of a semiconducting biradical oligomer with a motor-flexible design was demonstrated with a record-breaking PCE of 87% upon NIR-II excitation. This nanomedicine inherently features extraordinary photothermal stability, good biocompatibility, and excellent photoacoustic performance, contributing to high-contrast photoacoustic imaging in living mice and high-performance photothermal elimination of tumors. Moreover, a whole-cell vaccine based on a NIR-II nanomedicine with NIR-II-activated performance was further designed to remotely activate the antitumor immunologic memory and effectively inhibit tumor occurrence and metastasis in vivo, with good biosafety. Thus, this work paves a new avenue for designing NIR-II active semiconducting biradical materials as a promising theranostics platform and further promotes the development of NIR-II nanomedicine for personalized cancer treatment.