Abstract: Thermal and mechanical properties of yttrium tantalate (YTaO₄), a top coat ceramic of thermal barrier coatings (TBCs) for aeroengines, are enhanced by synthesizing Y_1−xTa_1−xM_2xO₄ (M = Ti, Zr, Hf; x = 0.06, 0.12, 0.18, 0.24) medium-entropy ceramics (MECs) using a two-step sintering method. In addition, the thermal conductivity, thermal expansion coefficients (TECs), and fracture toughness of MECs were investigated. An X-ray diffraction study revealed that the Y_1−xTa_1−xM_2xO₄ MECs were monoclinic, and the Ti, Zr, and Hf doping elements replaced Y and Ta. The variations in atomic weights and ionic radii led to disturbed atomic arrangements and severe lattice distortions, resulting in improving the phonon scattering and reduced thermal conductivity, with Y_1−xTa_1−xM_2xO₄ MECs (x = 0.24) exhibiting the lowest thermal conductivity of 1.23 W·m⁻¹·K⁻¹ at 900°C. The introduction of MO₂ increased the configurational entropy and weakened the ionic bonding energy, obtaining high TECs (10.4 × 10⁻⁶ K⁻¹ at 1400°C). The reduction in the monoclinic angle β lowered the ferroelastic domain inversion energy barrier. Moreover, microcracks and crack extension toughening endowed Y_1−xTa_1−xM_2xO₄ MECs (x = 0.24) with the highest fracture toughness of (4.1 ± 0.5) MPa·m^1/2. The simultaneous improvement of the thermal and mechanical properties of the MO₂ (M = Ti, Zr, Hf) co-doped YTaO₄ MECs can be extended to other materials.