Metastatic melanoma can be difficult to detect until at the advanced state that decreases the survival rate of patients. Several FDA‐approved BRAF inhibitors have been used for treatment of metastatic melanoma, but overall therapeutic efficacy has been limited. Lutetium‐177 (¹⁷⁷Lu) enables simultaneous tracking of tracer accumulation with single‐photon emission computed tomography and radiotherapy. Therefore, the codelivery of ¹⁷⁷Lu alongside chemotherapeutic agents using nanoparticles (NPs) might improve the therapeutic outcome in metastatic melanoma. Cellulose nanocrystals (CNC NPs) can particularly deliver payloads to lung capillaries in vivo. Herein, ¹⁷⁷Lu‐labeled CNC NPs loaded with vemurafenib ([¹⁷⁷Lu]Lu‐CNC‐V NPs) is developed and the therapeutic effect in BRAF V600E mutation‐harboring YUMM1.G1 murine model of lung metastatic melanoma is investigated. The [¹⁷⁷Lu]Lu‐CNC‐V NPs demonstrate favorable radiolabel stability, drug release profile, cellular uptake, and cell growth inhibition in vitro. In vivo biodistribution reveals significant retention of the [¹⁷⁷Lu]Lu‐CNC‐V NPs in the lung, liver, and spleen. Ultimately, the median survival time of animals is doubly increased after treatment with [¹⁷⁷Lu]Lu‐CNC‐V NPs compared to control groups. The enhanced therapeutic efficacy of [¹⁷⁷Lu]Lu‐CNC‐V NPs in the lung metastatic melanoma animal model provides convincing evidence for the potential of clinical translation for theranostic CNC NP‐based drug delivery systems after intravenous administration.