The use of pultruded glass-fiber reinforced polymer (GFRP) has increased in recent years, with important applications for footbridges, bridge decks, cooling towers, among other. The mechanical characterization of GFRP in terms of elastic properties is often carried out by means of destructive static tests specified in international standards, but it may be a quite tedious activity given the number of properties to be determined. An alternative approach consists in the use of non-destructive techniques based on dynamic testing, also known as experimental modal analysis (EMA). This work aims to identify the dynamic properties of thin-walled GFRP members regarding their natural frequencies, mode shapes and damping ratios. These experimental dynamic characteristics obtained for a C-channel beam are, then, used to determine specific material elastic properties back-calculated using numerical optimization process. It is shown that the properties determined through dynamic testing are in relatively good agreement with those obtained from classical destructive tests. This non-destructive technique is an alternative approach for the mechanical characterization of GFRP members, especially important for low cost in situ quality control.