High water consumption, together with the use of synthetic dyes and metallic mordant agents, contribute to the high environmental impact of the textile industry. Numerous investigations have focused on the search for more sustainable raw materials and processes for this sector. One of the most promising solutions is to look towards forest by-products as a sustainable source of fibrous raw materials to substitute plastic fibres and replace partially cotton. In addition, forest by-products could also be a good source of natural dyes and textile additives, replacing synthetic ones. The main by-product generated in the forestry industry is bark, derived from the debarking process. Pinus and eucalyptus are nowadays two of the most important tree species exploited by the forestry industry in southwestern Europe. This work investigates the application of Maritime pine (Pinus pinaster, Ait.) and the Eucalyptus (Eucalyptus Globulus) barks as a source of high polyphenolic content extracts, to be used as natural dyes in the textile industry. Extraction was performed with water in alkali conditions. The influence of the extraction conditions to obtain the extracts used as natural colorant and the dyeing conditions (pH, time, temperature, use of mordant) on the properties of the dyed textile sample was evaluated. It was shown that the pH and the extraction conditions used were the variables with the greatest influence on the final properties of the dyed textile. In addition, this work also demonstrated that it is possible to use the extracts obtained from both forest by-products as textile dyes without need for using any metallic mordant.