The facet of opticalfibers coated with nanostructures enables the development of ultraminiature and sensitive (bio)chemical sensors. The sensors reported until now lack specificity, and the fabrication methods offer poor reproducibility. Here,we demonstrate that by transforming the facet of conventional multimode opticalfibers onto plasmon resonance energy transfer (PRET) antenna surfaces, the specificity issues may be overcome. To do so, a low-cost chemical approach was developed to immobilize gold nanoparticles on the opticalfiber facet in a reproducible and controlled manner. Our nanosensors are highly selective as PRET is a nanospectroscopic effect that only occurs when the resonance wavelength of the nanoparticles matches that of the target parameter. As an example, we demonstrate the selective detection of picomolar concentrations of copper ions in water. Our sensor is 1000 times more sensitive than the state-of-the-art technologies. An additional advantage of our nanosensors is their simple interrogation; it comprises of a low-power light-emitting diode, a multimode opticalfiber coupler, and a miniature spectrometer. We believe that the PRET-basedfiber-optic platform reported here may pave the way for the development of a new generation of ultraminiature, portable, and hypersensitive and selective (bio)chemical sensors.