•Natural agar was firstly introduced into intumescent fire-retardant coatings on steel substrates in this study.•Only 1 % of agar addition could notably enhance the fire resistance and smoke suppression.•Agar addition promoted the formation of a notably compact and consistent carbon layer and a possible mechanism was proposed. To improve the fire resistance and smoke suppression performances of intumescent fire-retardant (IFR) coatings, natural agar was firstly introduced into IFR coatings on steel substrates in this study. It is found that the introduction of only small quantity of agar could notably enhance the fire resistance. The coating with 1.0 wt.% agar reduced the total heat release rate by 43.0 % and total smoke production of the coating by 57.3 % according to cone calorimetry test. It also exhibited significantly high water resistance. Observed through scanning electron microscopy, the forming char owns a notably compact and consistent carbon layer all through its heated process. Various analysis and characterization reveal that the agar introduction enhanced the char-forming ability of the coatings and significantly promoted the abundant CO2 release which should play a positive role in improving the function of vapor phase flame retardancy and the expansion ratio of the char. Further, the addition notably promoted the formation of the well cross-linked carbon layer and improved its graphitization degree. Benefited from these advantages, the coating with 1.0 wt.% agar obtained the excellent fire resistance and smoke suppression. This study provides a constructive and effective idea to improve the fire performances of IFR coating.