Kappa/ iota carrageenan was extracted from Chondracanthus chamissoi algae using hot water at 80°C (HC). The corrosion inhibition effect of HC on 2.25Cr-1Mo steel in 1 M HCl solution at 25, 45, and 65°C was studied using gravimetric tests and electrochemical studies. HC is a good corrosion inhibitor for 2.25Cr-1Mo steel in 1 M HCl; its inhibition efficiency improves as its concentration and the temperature of the acid medium increase. Electrochemical impedance spectroscopy (EIS) results reported an inhibition efficiency of 79.2% at 65°C with 1.5 g·L–1 HC. Potentiodynamic polarization (PDP) studies indicate that HC is a mixed-type inhibitor with mostly anodic inhibitory effects. The EIS results show an increase in Rct and a decrease in Cdl as HC concentrations increase, which infers the formation of a protective film on the steel that inhibits the corrosive process. The adsorption behavior of HC on 2.25Cr-1Mo steel fits the Langmuir adsorption isotherm mainly by chemisorption, as evidenced by a decrease in the activation energy in the presence of HC, an increase in the efficiency of inhibition of HC with increasing temperature, and a positive standard adsorption enthalpy. All experimental techniques, namely gravimetric, PDP, EIS, linear polarization resistance (LPR), and electrochemical frequency modulation (EFM), produce results that are in good agreement. Scanning electron microscopy and energy dispersive X-Ray spectroscopy (SEM/ EDS) investigations showed that HC adsorbs on 2.25Cr-1Mo steel, generating a protective layer that reduces surface damage caused by corrosive acid media. HC is a semi-refined carrageenan that is non-toxic and inexpensive due to its simple extraction in a hot aqueous medium. © 2023, Russian Association of Corrosion Engineers. All rights reserved.