This paper proposed a pitching wave energy converter with adjustable natural period (called C-WEC), which is achieved by moving the movable mass block. A prototype was fabricated and tested in a wave flume to study the performance of the C-WEC. Free decay tests demonstrated that the natural period of the prototype can be adjusted between 1.60 s and 3.30 s. The wave flume tests showed that for regular and irregular waves, the prototype can capture high mechanical power and exhibit large response amplitude operator (RAO) as well as high capture width ratio (CWR) across a wide range of wave periods, avoiding poor performance in non-resonant states. The Froude scaling law was used to assess the annual average power of a 1:16th full-scale C-WEC in real seas off the Canary Islands based on the measured mechanical power in irregular waves. The result showed that the annual average power of the C-WEC using tuning control can be up to 29.8 kW, an increase of 67.4% compared to the WEC with a constant natural period. The experimental results showed that the C-WEC is a promising pitching wave energy converter and its natural period adjustment method can be easily applied to other pitching WECs.