Next-generation high-efficiency concentrated solar power plants are envisioned to operate at temperatures as high as 1000°C exceeding the safe operational limit of existing metallic-based heat exchangers (HXs). Ceramics are the material of choice for extreme environments if their manufacturing challenges are overcome. Particularly, existing 3D-printed ceramic heat exchangers suffer from leakage issues through thin walls separating hot snd cold flow streams due to excessive usage of a non-volatile photopolymer content. Here, a milli-channel extrusion-based 3D-printed ceramic HX offering high-temperature strength combined with a low coefficient of thermal expansion is studied for extreme environments. The 3D-printed HX showed a high quality with no through-plane leakage due to limited organic non-volatile additives added. Thermo-hydraulic characteristics of the 3D-printed alumina HX are experimentally investigated over a wide range of hot inlet temperatures. Air was employed as the working fluid for both hot and cold sides. Experimental results showed the volume-based power density (VBPD) of the 3D-printed alumina HX is up to 8.2 MW/m³ at a hot and cold inlet temperature of 700 and 30°C, respectively. Additionally, experimental results indicated that, at a fixed air flow rate, the air pressure drop penalty increases with the hot inlet temperature owing to a rise in air viscosity. Insights gained from this study facilitate the design of innovative 3D-printed ceramic HXs with complex topologies and outstanding high-temperature durabilities under extreme environments.