This study investigates the effect of alkali-treated abaca fiber (Musa textilis Née) mass fraction (10 wt.%, 15 wt.%, and 20 wt.%) on the mechanical response and fracture morphology of unsaturated polyester composites fabricated via compression molding, compared against an untreated 20 wt.% abaca fiber composite as the control. Composite specimens were characterized according to ASTM D638 and ASTM D6110 standards, complemented by Scanning Electron Microscopy (SEM) analysis. Results demonstrated a progressive enhancement in tensile strength with increasing treated fiber fraction, rising from 10.90 ± 1.17 MPa for the untreated 20 wt.% control to 23.90 ± 2.90 MPa for the 20 wt.% alkali-treated composite, representing a 119.3% improvement attributable to surface modification. Young's modulus similarly increased from 188.98 ± 25.64 MPa (control) to 286.85 ± 13.36 MPa (20 wt.% treated). Impact energy absorption and toughness reached maximum values of 15.48 ± 2.72 J and 0.1190 ± 0.0200 J/mm² at 20 wt.% treated fiber loading, corresponding to improvements of 96.9% and 92.0% over the untreated control, respectively. SEM analysis confirmed that the 20 wt.% treated composite exhibited optimized interfacial bonding, characterized by minimal void content and a predominance of fiber fracture over pull-out, whereas the untreated control displayed dominant fiber pull-out due to weak interfacial adhesion caused by natural impurities. Alkali treatment combined with 20 wt.% fiber loading is identified as the configuration for lightweight structural applications.