Most plasmonic sensors are based on monitoring shifts of the plasmon resonance peak. Here, we report on a new approach that consists of monitoring nanospectroscopic absorption changes of the plasmon resonance spectrum of gold nanoparticles immobilized on the facet of a multimode optical fiber. The plasmon resonance peak was intentionally selected to match the absorption bands of a target biomolecule, which in our case was reduced Cytochrome c. The reduced state of Cytochrome c shows two absorption peaks ca. 520 and 550 nm. Such peaks were observed in the plasmonic spectrum of 40 nm-diameter gold nanoparticles that were excited with a white light LED. Our sensor exhibits a linear behavior for reduced cytochrome c concentrations up to 80 μM and a limit of detection of 60 nM. In spite of their simplicity, our sensors have comparable performance to other sophisticated optical and electrochemical methods commonly used to detect Cytochrome c. We believe that the approach here reported may open new alternatives for plasmonic sensing.