The antioxidant activity of rosemary (Rosmarinus officinalis) extract from different raw materials has been studied. Extracts were prepared from wild or drip-irrigated plants, as well as from the by-product resulting from the distillation of the aromatic essential oil. The radical scavenging activity of rosemary extracts was compared with that of antioxidants widely used in food, such as BHT and delta-tocopherol, using an optimization of the method based on the reduction of the radical 2,2-diphenyl-1-picrylhydrazyl (DPPH). The results pointed the excellent antioxidant activity of the crude fresh rosemary extracts, which was almost identical to that of pure delta-tocopherol, and higher than that of BHT; extracts prepared from distilled rosemary showed the lowest activity, although they are also of interest due to the low cost of the raw material. High performance liquid chromatography (HPLC) combined with diode array (DAD) and electrospray (ESI)-ion trap-MS detection was used to separate and identify the compounds present in the rosemary extracts. Rosmarinic acid, carnosic acid and seven of their terpene-type metabolites, and seven flavones were identified. The drying and/or distillation treatments used with the plant material strongly affected the content of the two compounds of higher antioxidant activity: rosmarinic acid and carnosic acid.