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Natural garnets are robust silicate minerals stable over large ranges of temperature and pressure that can provide useful geochemical constraints on petrogenetic conditions. The purpose of this study is to determine how the size of cation sites controls rare earth elements (REE) incorporation in mantle eclogitic garnets. Major and lanthanide element data in a suite of mantle-derived eclogite garnets were combined with new single-crystal structure refinements (SREFs) to examine the effects of major-element chemistry and site dimension on the incorporation of REE into the garnet structure. Several distinct trends are apparent. Pyrope-rich samples similar to mantle lherzolitic garnets are enriched in the smaller heavy rare earth elements (HREE). Almandine-rich garnets are also HREE-enriched, but low rare earth elements (LREE) values are lower than in the pyrope-rich garnets. Intermediate garnet compositions are more depleted in HREE and enriched in LREE (Ce, Nd, and Sm). Finally, Ca-rich garnets (50% grossular component) are depleted in LREE and HREE, but are enriched in MREE. Hence, the X site dimension does exert a crucial role in REE incorporation into the garnet structure. Crystal structure refinements provide further evidence of this influence.