Abstract: A flexible medium-entropy metallic glass (ME-MG) ribbon with a nominal composition of Co56Fe14B21Ta4Ti5 is developed via an entropy-driven design and a scalable melt-spinning technique. This approach reduces the cost associated with high Co content, mitigates environmental risks from metal leaching, and achieves an exceptional balance between catalytic activity, durability, and environmental safety for sustainable peroxymonosulfate (PMS) activation. The catalyst exhibits reliable adaptability across a wide pH range (3-11) conditions, various anions, and mixed dye matrices. Notably, it exhibits exceptional Rhodamine B degradation efficiency, retaining > 95% color removal over 200 consecutive cycles without structural failure. In contrast to Ta/Ti-free Co60Fe15B25 counterpart with rapid decay and severe metal leaching, the present ME-MG demonstrates enhanced cycling stability and suppressed ion release, confirming Ta/Ti’s critical role in stabilizing the surface and prolonging catalyst lifetime. This durability originates from a dynamically self-regulating surface architecture: a Fe/Co-rich transition layer and a Ta/Ti/O-rich passive layer that exfoliates to expose fresh active sites. It also arises from multiscale heterogeneous interfaces between the amorphous matrix and in-situ formed nanocrystals and Ti(O,N)-rich phases, which enhance electron transfer. Quenching experiments and electron paramagnetic resonance indicate a dominant singlet oxygen-mediated pathway during PMS activation, with minor radical participation. This work provides a distinct perspective to design eco-friendly ME-MG catalysts for water remediation.