This paper presents a novel method of estimating ocean waves by measuring current outputs from the Archimedes Wave Swing (AWS) wave energy converter (WEC). The wave period and height are crucial information to maximize the power output from the AWS. However, since the AWS is installed on the seabed, additional sensors and buoys are required to obtain the ocean wave information. The extended Kalman filter (EKF) is applied to obtain the states of the ocean wave by measuring the generator current in the α - β domain. As the EKF requires the Jacobian matrix of the system dynamic equations for the system matrix, simplified hydrodynamics of the floater including the Froude-Krylov excitation force and the domain α - β voltage equation of the generator is derived to develop the mathematical model of the AWS. In order to verify the performance of the estimator, a numerical simulation is performed and presented and it shows great agreement with the actual motion.