Investigations of reaction mixtures RE x (Au 0.79 Si 0.21 ) 100- x (RE = Y and Gd) yielded the compounds REAu 3 Si which adopt a new structure type, referred to as GdAu 3 Si structure ( tP 80, P 4 2 / mnm , Z = 16, a = 12.8244(6)/12.7702(2) Å, and c = 9.0883(8)/9.0456(2) Å for GdAu 3 Si/YAu 3 Si, respectively). REAu 3 Si was afforded as millimeter-sized faceted crystal specimens from solution growth employing melts with composition RE 18 (Au 0.79 Si 0.21 ) 82 . In the GdAu 3 Si structure, the Au and Si atoms are strictly ordered and form a framework built of corner-connected, Si-centered, trigonal prismatic units SiAu 6 . RE atoms distribute on 3 crystallographically different sites and each attain a 16-atom coordination by 12 Au and 4 Si atoms. These 16-atom polyhedra commonly fill the space of the unit cell. The physical properties of REAu 3 Si were investigated by heat capacity, electrical resistivity, and magnetometry techniques and are discussed in the light of theoretical predictions. YAu 3 Si exhibits superconductivity around 1 K, whereas GdAu 3 Si shows a complex magnetic ordering, likely related to frustrated antiferromagnets exhibiting chiral spin textures. GdAu 3 Si-type phases with interesting magnetic and transport properties may exist in an extended range of ternary RE-Au-Si systems, similar to the compositionally adjacent cubic 1/1 approximants RE(Au,Si) ∼6 .