Cellulosic waste has stood out as potential materials for obtaining cellulose, since this raw material is highly available in a wide variety of species. These can be agricultural, forestry, or industrial. Cotton is an agricultural material of high cellulose content and great technological and economic importance. Another source with great potential is the waste from the paper industry. Both materials generate large amounts of waste that were little explored and uncorrected disposed. This work proposes the recycling of these two types of residue through the production of nanocellulose (cotton waste (CW-N) and industrial wastes (IW-N)) and their characterization by Fourier transform infrared spectroscopy, dynamic light scattering, X-ray photoelectron spectroscopy, X-ray diffraction, morphological analysis, and thermal properties. The nanocelluloses (NCs) were incorporated in poly(lactic acid) matrix, and the composites were evaluated mechanically. After the isolation of the nanocelluloses, it was found that both materials showed similar physicochemical characteristics, such as chemical functional groups and atomic composition. However, the morphologies are very distinct: the CW-N is nanofibrillar, with mean diameter around 30 nm, and the IW-N is spherical and irregular, with radius varying from 30 to 100 nm, which can be associated with the different crystalline structure of the materials. The differences in the structure were evaluated through Rietveld Refinement, and the industrial residue showed the presence of impurities in large amounts, and an increase in the cellulosic content after the conversion into nanoscale. The biocomposites showed a significant increase in the mechanical results, with improvement in the tensile strength from 63 to 69 and 78 MPa to agricultural and industrial nanocelluloses, respectively, which is associated with the good stress transfer between the fillers and the matrix and possible interactions between the active sites of PLA and nanocelluloses. Graphic