Piezoceramics 0.99[(Bi0.5Na0.4K0.1)(Ti0.980Nb0.020)O3]–0.01(Ba0.7Sr0.3)TiO3 (BNKTN20–BST) were synthesized by a conventional solid-state reaction method, and temperature-dependent dielectric and ferroelectric studies were performed. The dielectric constant is well described by the law log(1/ε − 1/εm)γ=Clog (T − Tm) with a diffusive factor (γ) close to 1.7, which confirms that BNKTN20–BST exhibited a diffuse phase transition. Moreover, the behavior of real and imaginary permittivity versus logarithm of frequency was explained in detail in the three different ranges T<Td, Td<T<Tm, and T>Tm. In the range T<Td, ε decreases almost linearly with the logarithm of frequency due to the coexistence of ferroelectric and ferroelectric relaxor mixed phase. In the ranges Td<T Tm, ε shows a more complicated dependence on frequency, which is ascribed to the presence of a non-polar phase and to a loss of interaction between polar-regions. In Cole–Cole plots, there are mainly two overlapping semicircles above 300 °C, which correspond to grains and grain boundaries in the material. The activation energies (Ea) for grains and grain boundaries were calculated from the slope of the ln σdc versus 1000/T and were found to be 0.46 and 0.75 eV, respectively.