Summary: In this article, we have obtained necessary and sufficient conditions in terms of the canonical structure \(F \) on a semi-invariant submanifold of an almost contact manifold under which the submanifold reduces to a semi-invariant warped product submanifold. Moreover, we have proved an inequality for the squared norm of the second fundamental form and finally, an estimate for the second fundamental form of a semi-invariant warped product submanifold in a generalized Sasakian space form is obtained, which extends the results of Chen, Al-Luhaibi et al., and Hesigawa and Mihai in a more general setting.