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The structures and magnetic properties of the arsenic- and selenium-ligated dysprosium single-molecule magnets (SMMs) [Cp′3Dy(AsH2Mes)] (3-Dy), [(η5-Cp′2Dy){μ-As(H)Mes}]3 (4-Dy), [Li(thf)4]2[(η5-Cp′2Dy)3(μ3-AsMes)3Li] ([Li(thf)4]2[5-Dy]), and [(η5-Cp′2Dy){μ-SeMes}]3 (6-Dy) are described. The arsenic-ligated complexes 4-Dy and 5-Dy are the first SMMs to feature ligands with metalloid elements as the donor atoms. The arsenide-ligated complex 4-Dy and the selenolate-ligated complex 6-Dy show large anisotropy barriers in the region of 250 cm−1 in zero d.c. field, increasing to 300 cm−1 upon 5% magnetic dilution. Theoretical studies reveal that thermal relaxation in these SMMs occurs via the second-excited Kramers' doublet. In contrast, the arsinidene-ligated SMM 5-Dy gives a much smaller barrier of 23 cm−1, increasing to 35 cm−1 upon dilution. The field-dependence of the magnetization for 4 …