[en] Highlights: • Thermally stimulated relaxation was observed by dielectric spectroscopy and thermally stimulated depolarization current (TSDC) analyses. • Defect dipoles [(Ti_Ti^')-(V_O^{• •} )], in-grain and across-grain-boundary oxygen vacancies (V_O^{• •} ) were the main defect types in BaTi₄O₉. • (Ti_Ti^')-(V_O^{• •} ) and (V_O^{• •} ) were responsible for the observed thermally simulated relaxation at low and high temperature. • Microwave dielectric performance and defect behavior are closely related to the connections of TiO₆ octahedra in typical Ti-containing MWDCs. • A slow heating process gave the undoped BaTi₄O₉ ceramics with a high Q×f value of ~ 51 500 GHz. Relaxation phenomena in dielectrics are well known to be closely related to their defect chemistry. However, the relaxation behavior of microwave dielectrics has seldom been considered, despite these materials being an important member of the electroceramic family. Here, the relaxation behavior of the typical microwave dielectric ceramic barium tetratitanate (BaTi₄O₉) was investigated. Significant thermally stimulated relaxation was observed in both dielectric spectroscopy and thermally stimulated depolarization current (TSDC) measurements, and the correlation between relaxation and defect behavior was addressed. TSDC characterization revealed that defect dipoles [(Ti_Ti^')−(V_O^{• •} )] with activation energies (E_a) of 0.32–0.36 eV appeared at low temperature, while in-grain and across-grain boundary oxygen vacancies (V_O^{• •} ) appeared at higher temperature, yielding E_a of 0.30–0.48 eV and 0.65–1.01 eV, respectively. It was further believed that these two types of defects, (Ti_Ti^')−(V_O^{• •} ) and V_O^{• •} , were correspondingly responsible for the dielectric relaxation observed at low and high temperature. The relaxation behavior of several typical Ti-containing microwave dielectrics, such as BaTi₄O₉, TiO₂, MgTiO₃ and Ca_0.8Sr_0.2TiO₃, was compared, which suggests that microwave dielectric performance and defect behavior are closely related to the connections of TiO₆ octahedra in the materials.