A novel peptide, Cm39, was identified in the venom of the scorpion . Its primary structure was determined. It consists of 37 amino acid residues with a MW of 3980.2 Da. The full chemical synthesis and proper folding of Cm39 was obtained. Based on amino acid sequence alignment with different K channel inhibitor scorpion toxin (KTx) families and phylogenetic analysis, Cm39 belongs to the α-KTx 4 family and was registered with the systematic number of α-KTx 4.8. Synthetic Cm39 inhibits the voltage-gated K channel hK1.2 with high affinity (K = 65 nM). The conductance-voltage relationship of K1.2 was not altered in the presence of Cm39, and the analysis of the toxin binding kinetics was consistent with a bimolecular interaction between the peptide and the channel; therefore, the pore blocking mechanism is proposed for the toxin-channel interaction. Cm39 also inhibits the Ca-activated K2.2 and K3.1 channels, with K = 502 nM, and K = 58 nM, respectively. However, the peptide does not inhibit hK1.1, hK1.3, hK1.4, hK1.5, hK1.6, hK11.1, mK1.1 K channels or the hNa1.5 and hNa1.4 Na channels at 1 μM concentrations. Understanding the unusual selectivity profile of Cm39 motivates further experiments to reveal novel interactions with the vestibule of toxin-sensitive channels.