Telah dites efek karboksimetil kitosan (KMK) sebagai inhibitor korosi pada baja lunak dalam air gambut menggunakan metode berat hilang. Efisiensi inhibisi juga dikarakterisasi berdasarkan energi aktivasi reaksi korosi dengan dan tanpa KMK dan pembentukan lapisan film pasif pada permukaan logam dipelajari menggunakan Scanning Electron Microscope-Energy Dispersive X-Ray (SEM-EDX) dengan dan tanpa menggunakan KMK. Hasil menunjukkan bahwa efisiensi inhibisi korosi dipengaruhi oleh pH air gambut, waktu perendaman dan konsentrasi KMK. Inhibisi optimum yaitu 93,66%. terjadi pada pH 7 dan waktu perendaman 3 hari. Efisiensi inhibisi inhibitor ini meningkat dengan naiknya konsentrasi KMK. Inhibisi KMK pada permukaan baja mematuhi modifikasi persamaan isoterm adsorpsi  Langmuir dan diasumsikan terjadi melalui adsorpsi kimia pada permukaan logam dengan nilai DGoads -35,413 kJ mol-1. Nilai negatif DGoads menunjukkan bahwa adsorpsi KMK terjadi secara spontan pada permukaan baja lunak. Kata kunci: Air gambut, Baja lunak, Inhibitor korosi, Karboksimetil kitosan.  Carboxymethyl Chitosan as a Corrosion Inhibitor for Mild Steel in Peak Water Abstract Carboxymethyl chitosan (CMC) had been tested as a corrosion inhibitor for mild steel in peat water using weight loss method. The inhibition efficiency is also characterized based  on activation energies of corrosion reaction and the formation of passive film on the surface of the metal was studied using Scanning Electron Microscope-Energy Dispersive X-Ray (SEM-EDX) in the presence and absence of CMC. Result showed that the inhibition was influenced by the peat water pH, immersion time and CMC concentration. The optimum inhibition i.e. 93,66%  obtained at pH 7 and immersion time 3 days. The inhibition efficiency of this inhibitor increased with the increasing of CMC  concentration. The inhibition of CMC on the steel surface followed modified Langmuir isotherm equation and assumed to occur through chemical adsorption on the metal surface with DGoads -35.413 kJ mol-1 . The negative value of DGoads  indicated that the adsorption of CMC  onto the mild steel surface occured spontaneously. Keywords: Carboxymethyl chitosan, Corrosion inhibitor, Mild steel, Peat water.