Recently, utilization of ecofriendly procedures has been developed for synthesis of nanoparticles to avoid use of toxic chemicals and to achieve biological compatibility. Application of biosynthesis methods through the use of microorganisms, yeasts, plants or plant extracts is known as green synthesis. The ashes of burnt leaves and stems of Seidlitzia rosmarinus plant are called Keliab in Iran, mostly containing sodium and potassium carbonate. In this study, Keliab is introduced as a natural source for in situ synthesis of silver nanoparticles (Ag NPs) on cotton fabric. Absorption of carbonate ions on the surface stabilizes the nanoparticles and prevents agglomeration. Synthesis of Ag NPs on cotton fabrics was proved by X-ray diffraction (XRD), Fourier-transform infrared (FT-IR), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) analysis. Also ultraviolet-visible (UV-Vis) spectra of the remaining solutions of treated fabrics confirmed the synthesis of Ag NPs in the solution. Furthermore, the tensile strength, color change, and antibacterial activity of the treated cotton fabrics were investigated. Overall, the treated fabrics indicated excellent antibacterial properties against Staphylococcus aureus and Escherichia coli even at low Ag NP content with negligible change of color and tensile strength.