Invariant natural killer T (iNKT) cells recognize CD1d/glycolipid complexes and upon activation with synthetic agonists display immunostimulatory properties. We have previously described that the non-glycosidic CD1d-binding lipid, threitolceramide (ThrCer) activates murine and human iNKT cells. Here, we show that incorporating the headgroup of ThrCer into a conformationally more restricted 6- or 7-membered ring results in significantly more potent non-glycosidic analogs. In particular, ThrCer 6 was found to promote strong anti-tumor responses and to induce a more prolonged stimulation of iNKT cells than does the canonical α-galactosylceramide (α-GalCer), achieving an enhanced T-cell response at lower concentrations compared with α-GalCer both in vitro, using human iNKT-cell lines and in vivo, using C57BL/6 mice. Collectively, these studies describe novel non-glycosidic ThrCer-based analogs that have improved potency in iNKT-cell activation compared with that of α-GalCer, and are clinically relevant iNKT-cell agonists.