Rosmarinic acid (RA) is an important natural antioxidant used in the quality control of food and pharmaceutical products, and its rapid, selective, and sensitive determination is essential. In this work, an iron-based metal–organic framework (Fe-MOF) was used as a dual-mode sensor for RA determination. Ultraviolet (UV) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, Brunauer–Emmett–Teller (BET) surface area analysis, scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDX) were used. When deposited on a glassy carbon electrode, the Fe-MOF markedly enhanced the electroactive surface area and decreased the charge-transfer resistance toward the [Fe(CN)6]3−/4− redox couple compared with the bare electrode. Under optimized conditions in Britton–Robinson (BR) buffer (pH 3.5), the Fe-MOF-modified electrode exhibited a linear voltammetric response to RA over the range 2.0–101.8 µM, with a detection limit of 0.7 µM. In parallel, a fluorescence sensing platform based on Fe-MOF–RA interactions was established at 320-nm excitation and 450-nm emission, providing a linear range of 2.0–20.0 µM, with limits of detection and quantification of 2.53 and 8.35 µM, respectively. Both sensing modes showed good precision (RSD < 3%) and retained their response in the presence of common coexisting species such as rutin, caffeic acid, carnosic acid, ascorbic acid, Fe, and Mn ions. Additionally, the environmental sustainability of the methods was assessed using the Analytical GREEnness metric (AGREE). Application to Rosmarinus officinalis extracts by the standard addition method yielded recoveries close to 100%, demonstrating that Fe-MOF-based voltammetric and fluorescent methods are sensitive and selective alternatives for RA analysis in real herbal matrices.