Achieving high strength in porous zirconium carbide (ZrC)-based ceramics is notoriously challenging, primarily due to their high inherent porosity. Here, we present a creative, in situ synthesis strategy that utilizes anisotropic 3 mol% yttria-stabilized zirconia (YSZ) ice-templated foam as a reactive template. This novel approach yields a ZrC/YSZ composite foam with a high average axial compressive strength of 61.5 MPa at a porosity of ~70.9% at room temperature. The ZrC phase nucleates and grows for the first time within dense YSZ struts, not just on the surface. This unique reaction is intimately linked to the redistribution of Y³⁺ ions and the consequent tetragonal (t) to cubic (c) phase transformation in the YSZ matrix. Phase transformation in the matrix is a critical internal lever governing the mechanical properties, in some cases exceeding the influence of geometric factors. This research not only offers a new route to fabricate high-strength ultrahigh-temperature ceramic composite foams but also unveils the intricate interplay between their internal reaction chemistry and macroscopic mechanical strength.