This study investigates the impact of two drying methods — solar cabinet drying and open-air sun drying — on the retention of water-soluble vitamins (C, B₂, B₃, B₆, B₉) and flavonoids in Plantago major L. leaves. The goal is to determine which method ensures better preservation of nutraceuticals for functional food applications. Fresh Plantago leaves were divided and dried under two conditions: a solar cabinet dryer and natural open-air conditions. Both external and internal leaf zones were analyzed separately using HPLC-DAD at wavelengths of 250, 254, and 276 nm to quantify vitamin and flavonoid content. The solar dryer significantly outperformed open-air drying in preserving bioactive compounds. External tissues from solar-dried samples retained the highest level of vitamin C (971.9 mAU·s), while vitamin B₆ was best preserved in solar-dried internal tissues (512.4 mAU·s). Flavonoids such as rutin and degidrokvertsetin showed more pronounced peaks in solar-dried leaves, with degidrokvertsetin reaching 631.5 mAU·s — far higher than open-air equivalents (~437.7 mAU·s). The controlled temperature and enclosed design of the solar dryer minimized oxidative and photochemical losses compared to unregulated open-air drying. Solar cabinet drying is a superior method for retaining heat-sensitive vitamins and antioxidant flavonoids in Plantago major leaves. Its controlled microclimate leads to better quality and stability of nutraceuticals, making it more suitable for producing functional food ingredients than traditional open-air drying.