Cecropins (CECs) are insect venom-derived amphiphilic peptides with numerous pharmacological effects, including anti-inflammatory, antibacterial, antiviral, and anti-tumor activities. Cecropins induce tumor cell death by disrupting phospholipid membrane integrity. However, non-specific cytotoxicity and in vivo rapid degradation limit clinical application. Nanotechnologies provide novel strategies for tumor eradication, including nanocarriers that can precisely target drugs to tumor tissue. We report the fabrication of CEC-encapsulated zeolitic imidazolate framework 8 (ZIF-8) nanoparticles (CEC@ZIF-8 NPs) via the preparation of CEC@ZIF-8 NPs in pure water by one-pot stirring. This method yielded morphologically uniform NPs with 20 wt% drug loading capacity and 9% loading efficiency. The NP formulation protected CECs from proteasome degradation, enhanced peptide bioavailability, promoted HeLa tumor cell uptake, and increased antitumor efficacy compared to free CECs. In conclusion, this ZIF-8 encapsulation strategy may enhance the clinical applicability of CECs and other antitumor peptides.