We fabricated a GaInNAs semiconductor optical amplifier (SOA) by applying a facet coating to a buried-ridge-stripe GaInNAs laser. Due to a low reflectivity (<0.1%) and a wide bandwidth (70 nm) coating, Fabry–Perot (FP) modes of the GaInNAs laser were suppressed sufficiently, and thus a 1.3 µm traveling-wave GaInNAs SOA was realized for the first time. Peak chip gains of more than 9.6 dB and a 3-dB-gain bandwidth above 49 nm (9 THz) were obtained simultaneously with a cavity length between 600 µm and 900 µm. In addition, on/off ratios between 20 and 30 dB were obtained by switching the current on and off, which seems sufficient for the SOA to work as a switching device. With the temperature characteristics, we found that the ASE intensity and the gain coefficient of the GaInNAs SOA were much less dependent on temperature than those of conventional InP-based SOAs. These results demonstrate the superior temperature characteristics of the GaInNAs SOA compared with conventional InP-based SOAs.