A newly discovered advanced alloys known as high-entropy alloys (HEAs) have attracted research interest in the past two decades. The high-entropy alloys, more generally referred to as complex- concentrated alloys (CCAs), contain five or more principal elements were firstly proposed by Cantor et al. [1] and Yeh et al. [2-5] independently in 2004. In the present work, a non-equimolar HEA (AlCrCuFeNi) was developed. Similar to stainless steels, the proposed HEA system was expected to have promising corrosion behaviour in seawater due to the inclusion of passivating elements such as Cr and Ni in its composition. Comprehensive microstructural characterizations, such as x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and electron-backscattered diffraction (EBSD), were used to determine the microstructure of this new HEA. Further, potentiodynamic polarization, potentiostatic staircase method and electrochemical impedance spectroscopy were used to characterize the influence of temperature and chloride concentration on the corrosion behaviour of this newly developed HEA in a simulated marine environment. A direct correlation was found between temperature and chloride concentration with the HEA’s localized corrosion resistance. Additionally, the Point Defect Model [6] approach was employed to analyse the influence of the temperature and chloride concentration upon the properties of the passive film formation over the alloy surface in seawater. The Point Defect Model revealed that the studied HEA exhibited a better local corrosion resistance than conventional martensitic stainless steel UNS S40300 when temperature is below 60 ℃. Moreover, the grain size of the studied HEA varied significantly after grain refinement process. The corrosion properties of the different grain size HEAs were also characterized and it was revealed that its corrosion resistance can be enhanced by decreasing the grain size in this HEA system.