The forgotten topological index denoted by F(G) of a graph G = (V, E) is defined as follows: \(F (G) ={\sum }_{i=1}^{n}{d}_{v}^{3}\) where dv denotes the degree of the vertex v. We extend the notion of forgotten i=1 topological index to signed graphs and introduce the MS-index of a signed graph. Moreover, we determine the forgotten topological index for the tensor product, Cartesian product, lexicographic product, strong product, symmetric difference, and the joint of the graphs G1 and G2 in terms of the forgotten topological index, the first Zagreb index, and the MS-index of signed graphs Σ1 = (G1, σ1) and Σ2 = (G2, σ2), along with their sizes and orders.
