Sulfur cinquefoil (Potentilla recta L.), a perennial forb native to the eastern Mediterranean region of Eurasia, is a major noxious weed on rangelands of the northwestern United States and southwestern Canada. We assessed targeted sheep grazing to suppress sulfur cinquefoil in a 2-yr rangeland field experiment in northwestern Montana. We evaluated targeted sheep grazing with and without protein-energy supplementation (37 g crude protein [CP] sheep^-1 d^-1 and 0.17 kg total digestible nutrients [TDN] sheep^-1 d^-1) during late June (sulfur cinquefoil in early flowering stage) and mid-July (sulfur cinquefoil in late flowering—early seedset stage). Sheep readily consumed sulfur cinquefoil stems, leaves, flowers, and developing seed heads, with or without supplementation. Sulfur cinquefoil comprised the largest proportion of sheep diets during both late June and mid-July, averaging 46%, but more sulfur cinquefoil dry matter (DM) was consumed by sheep during mid-July (0.6 vs. 1.0 kg DM sheep^-1 d^-1 in June vs. July, respectively). Supplementation did not increase DM intake (DMI) of sulfur cinquefoil, nor did supplementation improve the nutritive quality of sheep diets. We also documented that 1) targeted sheep grazing achieved heavy utilization of sulfur cinquefoil (67%) while keeping perennial graminoid use light to moderate (18–41%); 2) targeted sheep grazing reduced viable seed production of sulfur cinquefoil by 97% in June-grazed paddocks and 95% in July-grazed paddocks; and 3) targeted sheep grazing reduced sulfur cinquefoil yield the next summer by 41% in June-grazed paddocks and 47% in July-grazed paddocks without decreasing yield or plant community composition of perennial graminoids. We conclude that supplemented or nonsupplemented targeted sheep grazing applied in either late June or mid-July can effectively suppress sulfur cinquefoil. Sheep nutrition and sulfur cinquefoil DMI will be optimized by targeted sheep grazing applied during mid-July.