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The proper disposal of cow dung waste has always been a great challenge for the agricultural sector in China and around the world. In this work, the cow dung was processed into a cheap renewable cow dung fiber (CDF) and applied into cementitious composites. The physicochemical properties of raw and alkali pretreated CDF were analyzed by thermogravimetry, Fourier transform infrared spectroscopy, X-ray diffraction, single fiber tensile test and scanning electron microscopy. The strength and autogenous shrinkage of cementitious composites reinforced with the CDF were measured. The results showed that alkali pretreatment dissolved the amorphous components (i.e., hemicellulose and lignin) and enhanced the crystallinity of cellulose. The pretreatment of CDF by 1 mol/L NaOH solution led the 3 d′s compressive and splitting tensile strength of the CDF reinforced cementitious composites to increase by 21.8% and 12.5%, respectively. The presence of CDF, with or without pretreatment enabled to inhibit the development of autogenous shrinkage of cementitious paste. This work is expected to promote an alternative application for the recycling of cow dung waste in sustainable building materials. • The cow dung was processed into the cheap renewable cow dung fibers. • Alkali pretreatment was implemented on the cow dung fiber. • Cow dung fiber reinforced cementitious composites was prepared. • The crystallinity of cow dung fiber was increased after alkali pretreatment. • The properties of cementitious composites were improved by adding pretreated cow dung fiber. [ABSTRACT FROM AUTHOR]