The research about harmonic analysis associated with Jacobi expansions carried out in [1] and [3] is continued in this paper. Given the operator J (α,β) = J (α,β) − I, where J (α,β) is the three-term recurrence relation for the normalized Jacobi polynomials and I is the identity operator, we define the corresponding Littlewood-Paley-Stein g (α,β) k -functions associated with it and we prove an equivalence of norms with weights for them. As a consequence, we deduce a result for Laplace type multipliers.