This study investigated the effects of rosemary dietary supplementation on the rumen degradability of alfalfa hay, the ruminal bacterial population and rumen metabolism in sheep. The experiment was designed as a 4 × 4 Latin square balanced for carry-over effects using four ruminally cannulated sheep fed a basal diet consisting of 1.5 kg/day of alfalfa hay supplemented (400 g/day) with one of four different concentrates as follows: (1) control (CTR); (2) CTR with the addition of 1.75% rosemary essential oil adsorbed on an inert support (EO); (3) CTR with the addition of 2.50% dried and ground rosemary leaves (RL); and (4) same as (3), except that rosemary leaves were included in the concentrate before pelleting (RL pellet). No effects were recorded in the volatile fatty acid or lactic acid production. Ruminal ammonia concentration tended to be lower (P = 0.077) with the RL pellet diet. The crude protein degradability, after incubation times of 2 and 48 h, decreased (P < 0.001) in sheep fed with the RL pellet diet. The dry matter degradability followed the same trend, with lower (P < 0.001) rates of disappearance resulting from the RL pellet diet after 24 h of incubation. The pelleting process may influence the availability of the active compounds of rosemary leaves, allowing greater antibacterial activity. Supplementation of sheep diets with rosemary leaves could potentially be used to manipulate ruminal degradation patterns.