Seaweed waste (SWW) is a residue or by-product from the filtration step of the agar extraction process, and it has been explored as an inexpensive and effective filler for incorporation by melt blending into a poly(lactic acid) (PLA) matrix. PLA-SWW biocomposites were manufactured with various contents of SWW (0, 5, 10, 15, and 20 wt %) using a sheet extrusion process. PLA was functionalized with maleic anhydride (MAH) by reactive extrusion using dicumyl peroxide (DCP) as an initiator, and it was extruded using 0, 5, and 20 wt %. SWW content. The mechanical, thermal, structural, and morphological properties of the processed biocomposites were investigated. Regarding the mechanical behavior, a slight increase in the tensile modulus was observed at low SWW content. The thermal properties indicated that the rigid amorphous phase content was enhanced in the biocomposites. This work suggests that SWW can be used as filler to develop environmental friendly biocomposites.