•Natural dye extracted from sawdust of Pterocarpus indicus Willd. using ultrasound.•Dyeing and mordanting parameters were optimized.•Pretreatment of cotton and silk improved colour depth, fastness and UV protection.•Timber industry waste can be upcycled as a resource for natural dye. Exploitation of synthetic dyes in a variety of manufacturing sectors including textiles poses serious problems to the environment. Natural dyes are becoming vital alternatives to toxic synthetic dyes, however, with limitations such as availability and process standardization. Herein, we explore the dyeing potential of a sawdust, Pterocarpus indicus Willd., which is widely available as a waste from timber industry in South Asia and other parts of the world. Ultrasound assisted extraction method was carried out to extract natural dye from the sawdust. The extract was used to dye cotton and silk fabrics using various metallic and natural mordants such as alum, stannous chloride, copper sulphate, gallnut, pomegranate rind and gooseberry and the process was optimized. The effect of different pretreatments such as chitosan and myrobalan on dyeing fabrics was also studied. Colour properties of the naturally dyed fabrics such as colour strength, colour co-ordinates and various fastness were assessed. The natural mordants such as gallnut, pomegranate rind and gooseberry provided comparable fastness property to metallic mordants, thereby suggesting the use of the extracted dye with natural mordants for exclusive eco-friendly dyeing. We also demonstrate that chitosan and myrobalan pretreatments significantly enhance the colour strength, colour fastness and UV protection properties of naturally colored cotton and silk fabrics. Overall, the results suggest copper sulphate mordanting with chitosan pretreatment and stannous chloride mordanting with myrobalan pretreatment as the best combination for cotton and silk fabrics, respectively considering the color fastness and UV protection properties. The results provide ample scope for the upcycling of timber industry waste into a natural dye for eco-friendly coloration of fabrics.