Rosmarinus eriocalyx (rosemary or Elyazir) is an endemic species growing in arid steppe and rocky mountain in the South-West Algeria. This plant is well known in Algeria and Morocco due to its medicinal properties. However, little is known about its phytochemical composition. For this purpose, natural antioxidant compounds from R. eriocalyx were recovered by solid-liquid extraction and characterized by reversed-phase high-performance liquid chromatography coupled to quadrupole-time-of-flight mass spectrometry using negative and positive ionization modes. This analytical methodology enabled the characterization of 101 compounds, which were distributed in five major categories namely hydroxycinnamic acid derivatives, hydroxybenzoic acid derivatives, flavonoids, phenolic diterpenes and phenolic triterpenes. Moreover, the studied extract generally showed free radical-scavenging and reductive abilities in the range of butylated hydroxyanisole, butylated hydroxytoluene, α-tocopherol, and ascorbic acid. Therefore, the result suggests that the aqueous-methanolic extract of R. eriocalyx could serve as a potential source of antioxidants.