Ag(Nb1−xTax)O3 (x = 0–0.8) ceramics were prepared using a solid-state reaction method, and a single-phase perovskite was identified using XRD for dense ceramics with x = 0–0.65. However, small Ag precipitates and microstructural inhomogeneities were observed using BSE-SEM and EDS. There are four kinds of dielectric abnormalities at RF frequencies, including a new abnormality that has a strong relaxation in the dielectric loss. A ferrielectric-like dielectric tunability was observed at 20–80 °C for x = 0 and 0.2, while the ceramic with x = 0.5 exhibited an antiferroelectric-like dielectric tunability. Furthermore, a transition behaviour was observed for x = 0.35, indicating that the ferrielectric–antiferroelectric transition does not correspond to the M1–M2 phase transition. The microwave dielectric constant of Ag(Nb1−xTax)O3 ceramics reaches its maximum value at x = 0.35, while the Qf value and the temperature coefficient of resonant frequency increase monotonously with the value of x.