Cathepsin L (CTSL) is a cysteine protease involved in a variety of physiological and pathological processes. Potent inhibitors against CTSL have long been sought for drug development. Due to insufficient specificity and suboptimal pharmacological properties for current CTSL inhibitors, novel agents are still required for selectively blocking CTSL activity. Here we generated a humanized antibody inhibitor of CTSL by genetically fusing the inhibitory propeptide of procathepsin L to the N-terminus of the light chain of a humanized antibody. The resulting antibody fusion could be stably expressed and displays highly potent inhibition activity and specificity toward CTSL. This work demonstrates a new approach for the rapid generation of antibody inhibitors of CTSL. It can possibly be extended to create inhibitory antibodies targeting other cathepsin proteases, providing novel research and therapeutic tools.