Offshore structures are subject to severe environmental conditions and require high operating and maintenance costs. At the design stage of an offshore structure, it is necessary to perform load analysis and to consider representative environmental conditions characterized by statistical models. However, many available joint distribution models of the environmental parameters can only describe the correlation of these parameters in a very restricted form. The use of simple probabilistic models without correctly addressing their correlation may lead to significant bias in the reliability analysis. Here, the correlation between three offshore environmental parameters including the significant wave height, wave peak period, and mean wind speed is described by copula. The copula density functions and theoretical derivations of copula correlation parameters using actual sea state data are provided for general applications of reliability analysis of offshore structures. Hindcast data of two representative sites are used to fit the best copula. The developed copula-based joint distribution can be used for accurate reliability analysis of offshore structures considering long-term fatigue loads and extreme responses.