Fluorescence imaging in the shortwave infrared (SWIR) window enables deeper tissue and higher-contrast visualization of pathological events compared to that in the conventional near-infrared (NIR) window. However, specific SWIR detection of biomarkers of interest remains challenging because few fluorophore scaffolds are available to be further modified into biomarker-activatable SWIR probes. Herein, we employ a highly efficient approach to synthesize six hemicyanine derivatives with emissions extending to the SWIR region and structural amenability for construction of activatable probes. The bathochromic shifts of these dyes are driven by the extension of the central polymethine bridge and the substitution of the xanthene endocyclic oxygen atom with a heavier sulfur atom. Among all fluorophores, HCS6, featuring three central methine-methine bonds, has the longest emission maximum at 1010 nm along with superior photostability and high SWIR brightness. These properties enable HCS6 to achieve lung imaging in haired mouse models without the need for shaving. Furthermore, HCS6 is constructed into an activatable probe that selectively turns on its SWIR fluorescence in response to a cancer-associated biomarker, allowing for the highly sensitive identification of metastatic lung lesions in unshaved mice. These hemicyanine scaffolds offer a promising platform for developing versatile activatable probes, highlighting their potential for SWIR microscopy and deep-tissue biological imaging.