Microwave　dielectric　materials　are　of　great　interest　due　to　their　applications　in　communication　technology.　The　intrinsically　low　dielectric　permittivity　(generally　less　than　100)　of　traditional　microwave　dielectric　materials　has　limited　their　capability　in　reducing　the　device　size　and　developing　various　applications.　In　this　paper,　we　report　a　microwave　dielectric　material,　(La　+　Nb)　co-doped　BaSnO3,　which　exhibits　both　frequency-　and　temperature-independent　colossal　permittivity　(　＞　103)　over　the　frequency　range　from　10　Hz　to　microwave　region　(∼1　GHz)　while　retaining　the　ultra-low　dielectric　loss　of　4　×　10-4,　equivalent　to　a　quality　factor　Qf　(GHz)　∼2500.　Systemic　defect　analysis　and　density　functional　theory　calculations　suggest　that　negatively　charged　La　and　positively　charged　Nb　octahedra　are　correlated　adjacent　to　each　other　along　the　[110]　direction,　forming　defect-dipole　clusters,　which　lead　to　their　microwave　dielectric　properties.　This　work　presents　insights　on　the　development　of　microwave　dielectric　materials　that　offer　many　potentials　for　microwave　dielectric　devices　and　their　associated　applications　in　future　communication　technology.　©　2021　American　Chemical　Society.