We introduce the notion of complex \(G_2\) manifold \(M_{\mathbb C}\), and complexification of a \(G_2\) manifold \(M\subset M_{\mathbb C}\). As an application we show the following: If \((Y,s)\) is a closed oriented 3-manifold with a \(Spin^{c}\) structure, and \((Y,s)\subset (M, \varphi)\) is an imbedding as an associative submanifold of some \(G_2\) manifold (such imbedding always exists), then the isotropic associative deformations of \(Y\) in the complexified \(G_2\) manifold \(M_{\mathbb C}\) is given by Seiberg-Witten equations.