This paper presents the results of a comprehensive non-destructive testing (NDT) campaign carried out on the penstock system of a major hydropower plant in Southeastern Europe. Conducted in May 2025, the inspection aimed to evaluate the structural integrity of five steel penstock units using a combination of visual testing (VT), ultrasonic testing (UT), and magnetic particle testing (MT). The assessment identified several critical issues, including external corrosion, pitting, wall thickness reduction, and internal weld defects. Notably, localized wall thinning of up to 30% was recorded in high-risk zones, and ultrasonic testing revealed lack-of-root-fusion flaws in multiple weld segments. Surface cracking was also detected via MT, with the most severe indication measuring 700 mm in length. Rust mapping helped visualize corrosion severity and distribution, guiding future maintenance strategies. The results underscore the urgent need for structural rehabilitation and demonstrate the value of periodic NDT programs in extending the lifespan and operational safety of aging hydropower infrastructure.