Enhancing Streptomyces cellulase production by supplying lignocellulose biomasses has been poorly investigated so far. In this research the biosynthesis of Streptomyces roseochromogenes ATCC13400 cellulases was increased for the first time by addition of a cellulose fraction (2.5 g·L−1) to the growth medium, isolated from the marine origin Posidonia oceanica sea balls, generally called egagropili.. In shake flasks the cellulase production increased of 4.3 folds, compared to the control, up to 268 U·L−1 in 72 h, with a productivity of 3.7 U·L−1·h−1, while in batch it was further enhanced up to 347 U·L−1 in 45 h with a doubled productivity of 7.7 U·L−1·h−1 A downstream protocol was set up by coupling two ultrafiltration steps on 10 and 3 kDa membranes to recover the enzymes from the supernatant. A pool of three cellulases, having molecular weights between 115 and 47 kDa, was recovered. The optimal conditions for their enzymatic activity were 60 °C and pH 5.0, and they showed CMCase, FPase and β-glucosidase action. In conclusion, S. roseochromogenes might be considered a new cell factory for cellulase biotechnological production that might be enhanced by using the cellulose from egagropili, a well-known marine origin plant waste, as the substrate.