An optical oxygen sensor based on an EuIII complex/polystyrene (PS) composite nanofibrous membrane is prepared by electrospinning. The emission intensity of [Eu(TTA)3(phencarz)] (TTA=2‐thenoyltrifluoroacetonate, phencarz=2‐(N‐ethylcarbazolyl‐4)imidazo[4,5‐f ]1,10‐phenanthroline) decreases with increasing oxygen concentration, and thus the [Eu(TTA)3 (phencarz)]/PS composite nanofibrous membranes can be used as an optical oxygen‐sensing material based on emission quenching caused by oxygen. Elemental analysis, UV/Vis absorption spectra, scanning electron microscopy (SEM), fluorescence microscopy, luminescence‐intensity quenching Stern–Volmer plots, and excited‐state decay analysis are used to characterize the obtained oxygen‐sensing materials. A high sensitivity (I N2/I O2) of 3.38 and short response and recovery times (t ↓=5.0, t ↑=8.0 s) are obtained. These results are the best values reported for oxygen sensors based on EuIII complexes. The high surface area‐to‐volume ratio and porous structure of the electrospun nanofibrous membranes are taken to be responsible for the outstanding performance.