Rare-earth phosphates (REPO<sub>4</sub>) are regarded as one of the promising thermal/environmental barrier coating (T/EBC) materials for SiC<sub>f</sub>/SiC ceramic matrix composites (SiC-CMCs) owing to their excellent resistance to water vapor and CaO–MgO–Al<sub>2</sub>O<sub>3</sub>–SiO<sub>2</sub> (CMAS). Nevertheless, a relatively high thermal conductivity (<i>κ</i>) of the REPO<sub>4</sub> becomes the bottleneck for their practical applications. In this work, novel xenotime-type high-entropy (Dy<sub>1/7</sub>Ho<sub>1/7</sub>Er<sub>1/7</sub>Tm<sub>1/7</sub>Yb<sub>1/7</sub>Lu<sub>1/7</sub>Y<sub>1/7</sub>)PO<sub>4</sub> (HE (7RE<sub>1/7</sub>)PO<sub>4</sub>) has been designed and synthesized for the first time to solve this issue. HE (7RE<sub>1/7</sub>)PO<sub>4</sub> with a homogeneous rare-earth element distribution exhibits high thermal stability up to 1750 ℃ and good chemical compatibility with SiO<sub>2</sub> up to 1400 ℃. In addition, the thermal expansion coefficient (TEC) of HE (7RE<sub>1/7</sub>)PO<sub>4</sub> (5.96×10<sup>−6</sup> ℃<sup>−1</sup> from room temperature (RT) to 900 ℃) is close to that of the SiC-CMCs. What is more, the thermal conductivities of HE (7RE<sub>1/7</sub>)PO<sub>4</sub> (from 4.38 W·m<sup>−1</sup>·K<sup>−1</sup> at 100 ℃ to 2.25 W·m<sup>−1</sup>·K<sup>−1</sup> at 1300 ℃) are significantly decreased compared to those of single-component REPO<sub>4</sub> with the minimum value ranging from 9.90 to 4.76 W·m<sup>−1</sup>·K<sup>−1</sup>. These results suggest that HE (7RE<sub>1/7</sub>)PO<sub>4</sub> has the potential to be applied as the T/EBC materials for the SiC-CMCs in the future.