The flammability of cotton is a critical concern, particularly in home textiles and apparel. This study developed an eco-friendly flame-retardant coating by combining sodium alginate (SA) and dicalcium phosphate (DCP). DCP forms phosphoric acid when dissolved in hydrochloric acid. It promotes the carbonization of cellulose and interferes with the combustion chain reaction. In addition, SA induced the formation of a stable char layer, as shown in SEM micrographs. This char layer provides thermal insulation and a barrier to oxygen. As a result, the coated cotton fabric with SA and DCP was able to self-extinguish in 1 s. However, all coated fabrics exhibited significantly higher stiffness values than uncoated cotton (CT). The CT showed low flexural rigidity (87 mg-cm weft, 227 mg-cm warp). The SA coating slightly increased stiffness of the fabric, probably due to film formation on the fiber surface. The CTDCP showed higher values, and the CTSA-DCP exhibited the highest stiffness (1333 mg-cm weft, 2661 mg-cm warp). It is probably due to the partial cellulose degradation by acid and possible crosslinking in the SA layer. These results demonstrate the potential of DCP and SA coating for enhancing flame-retardancy in cotton textiles. The significantly improved flame retardant performance with its flexural rigidity suggests that the SA–DCP system may be suitable for applications where increased fabric stiffness is acceptable, such as in protective clothing, home furnishings, or industrial textiles.