A filter device includes input and output terminals, first and second ground electrodes opposed to each other, and a resonator connected to one of the input and output terminals. The resonator includes a third ground electrode, and first, second, and third resonant portions. The third ground electrode is between and connected to the first and second ground electrodes. The first resonant portion is between the first and third ground electrodes, and connected to the third ground electrode and one of the input and output terminals. The second resonant portion is between the first and third ground electrodes, and connected to the third ground electrode. The third resonant portion is between the second and third ground electrodes, and connected to the third ground electrode.