A simple all-fiber sensor based on nested fiber balloon rings (FBRs) for simultaneous refractive index (RI) and temperature measurement is proposed and demonstrated. The FBR is easily fabricated by properly bending a section of single-mode fiber. The FBR with a smaller bending radius is embedded in a bigger one. Because the two separate interference dips have different RI and temperature sensitivities, which are formed by individual FBRs, the RI and temperature can be simultaneously measured by interrogating the wavelength shifts of the two interference dips. This sensor has the advantages of convenient fabrication, economical materials, probe structure, and high sensitivity and can be a candidate for simultaneous measurement of RI and temperature.