This study analysed the total content of phenolic compounds in roots and rhizomes extracts from two populations of Valeriana carnosa Sm. (Caprifoliaceae) at three phenological stages. Total phenolic content was determined through the Folin-Ciocalteu method, which ranged between individuals from 3.56 to 11.68 mg GAE/g of dry sample. Antioxidant activity was determined using the stable radical 2,2-diphenyl-1-picrylhydrazyl (DPPH), which showed a significant positive correlation between antioxidant activity and total phenolic content. We tentatively identified 18 phenolic compounds by HPLC-MS, mostly phenolic acids, one of which was present only in the Hoya population at the vegetative stage and one only in the Piltri population at the flowering and fruiting stages. Phenolic compounds in subterranean organs of V. carnosa vary qualitatively (between populations/stages) and quantitatively at intra- and inter-population level at different phenological stages. In both populations, on average a higher phenolic content and antioxidant activity were recorded at the flowering and fruiting stages.