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Abstract Let G be a simple graph with n vertices and ( 0 , 1 ) $(0,1)$ -adjacency matrix A. As usual, S ( G ) = J − 2 A − I $S(G)=J-2A-I$ denotes the Seidel matrix of the graph G. Suppose θ 1 , θ 2 , … , θ n $\theta_{1}, \theta_{2},\ldots, \theta_{n}$ and λ 1 , λ 2 , … , λ n $\lambda_{1}, \lambda_{2},\ldots, \lambda_{n}$ are the eigenvalues of the adjacency matrix and the Seidel matrix of G, respectively. The Estrada index of the graph G is defined as ∑ i = 1 n e θ i $\sum_{i=1}^{n} e^{\theta_{i}}$ . We define and investigate the Seidel-Estrada index, S E E = S E E ( G ) = ∑ i = 1 n e λ i $SEE=SEE(G)=\sum_{i=1}^{n} e^{\lambda_{i}}$ . In this paper the basic properties of the Seidel-Estrada index are investigated. Moreover, some lower and upper bounds for the Seidel-Estrada index in terms of the number of vertices are obtained. In addition, some relations between S E E $SEE$ and the Seidel energy E s ( G ) $E_{s}(G)$ are presented.