Chemiluminescence (CL) sensing without external excitation by light and autofluorescence interference has been applied to high‐contrast in vitro immunoassays and in vivo inflammation and tumor microenvironment detection. However, conventional CL sensing usually operates in the range of 400–850 nm, which limits the performance of in vivo imaging due to serious light scattering effects and signal attenuation in tissue. To address this challenge, a new type of CL sensor is presented that functions in the second near‐infrared window (NIR‐II CLS) with a deep penetration depth (≈8 mm). Successive CL resonance energy transfer (CRET) and Förster resonance energy transfer (FRET) from the activated CL substrate to two rationally designed donor‐acceptor‐donor fluorophores BTD540 and BBTD700 occurs. NIR‐II CLS can be selectively activated by hydrogen peroxide over other reactive oxygen species (ROSs). Moreover, NIR‐II CLS is capable of detecting local inflammation in mice with a 4.5‐fold higher signal‐to‐noise ratio (SNR) than that under the NIR‐II fluorescence modality.