We study fully supersymmetric AdS 6 vacua of half-maximal N = (1, 1) gauged supergravity in six space-time dimensions coupled to n vector multiplets. We show that the existence of AdS 6 backgrounds requires that the gauge group is of the form G × G ⊂ SO(4, n) where G ⊂ SO(3, m) and G ⊂ SO(1, n − m). In the AdS 6 vacua this gauge group is broken to its maximal compact subgroup SO(3) × H × H where H ⊂ SO(m) and H ⊂ SO(n − m). Furthermore, the SO(3) factor is the R-symmetry gauged by three of the four graviphotons. We further show that the AdS 6 vacua have no moduli that preserve all supercharges. This is precisely in agreement with the absence of supersymmetric marginal deformations in holographically dual five-dimensional superconformal field theories.