Soil contamination by microplastics is an increasing environmental concern due to the high persistence of low-density polyethylene (LDPE). This study investigated the biodeterioration of LDPE microplastics by dark septate endophytic (DSE) fungi in a minimal salt medium, with LDPE provided as the primary carbon-containing substrate. Five DSE isolates, namely Pleosporales sp. (BM2 and Mg3) and Cladosporium sp. (EC2, Mg2, and PNS), were incubated with sterile LDPE microplastics (250–425 µm) for 60 days at 25–28 °C. Biodeterioration performance was evaluated through mass loss measurements, Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM), with an additional dose-dependent analysis (0–2 g LDPE) conducted for the best-performing isolate. All isolates were able to colonize LDPE particles and induce oxidative deterioration of the polymer surface. The highest mass loss was observed for Cladosporium sp. strain PNS (35.0 ± 2.0% of the initial mass). FTIR analysis revealed a reduction in C–H stretching bands (2915–2848 cm⁻¹) and the emergence of O–H (3300–3500 cm⁻¹) and C=O (1700–1715 cm⁻¹) functional groups, indicating oxidative modification of the polymer structure. SEM observations further confirmed surface deterioration, including cracks and fragmentation, which intensified with increasing LDPE dose. Collectively, these findings demonstrate the potential of DSE fungi, particularly Cladosporium sp. strain PNS, to initiate LDPE microplastic biodeterioration and support their prospective application in sustainable, soil-oriented microplastic remediation strategies.