Cellulose nanofiber is the world’s most advanced biomass material. Most importantly, it is biodegradable. In this work, nanofibers were obtained from pineapple leaves, a large solid waste in Colombia, using a combined extraction method (chemical procedures and ultrasound). The native fibers were bleached, hydrolyzed, treated with ultrasound, and characterized by scanning electron microscopy (SEM), infrared analysis (FTIR), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). As a comparison, a commercial microcrystalline cellulose sample was analyzed, which demonstrated the efficiency of cellulose extraction. The nanofibers had a diameter and a length of 18 nm and 237 nm, respectively, with a maximum degradation temperature of 306 °C. The analysis showed the efficiency of acid treatment combined with ultrasound to obtain nanofibers and confirmed that pineapple residues can be valorized by this method. These results indicate that lignocellulosic matrices from pineapple leaves have potential application for obtaining polymeric-type composite materials. Due to their morphology and characteristic physical properties, the cellulose nanofibers obtained in this work could be a promising material for use in a wealth of fields and applications such as filter material, high gas barrier packaging material, electronic devices, foods, medicine, construction, cosmetics, pharmacy, and health care, among others.