Decentralized adaptive control design for a class of large-scale interconnected nonlinear systems with unknown interconnections is considered in this paper, The motivation behind this work is to develop decentralized control for a class of large-scale systems which do not satisfy the matching condition requirement. To this end, large-scale nonlinear systems transformable to the decentralized strict feedback form are considered, Coordinate-free geometric conditions under which any general interconnected nonlinear system can be transformed to this form are obtained, The interconnections are assumed to be bounded by polynomial-type nonlinearities. However, the magnitudes of the nonlinearities are unknown, Global stability and asymptotic regulation are established using classical Lyapunov techniques, The controller is shown to maintain robustness for a wide class of systems obtained by perturbations in the dynamics of the original system, Furthermore, appending additional subsystems does not require controller redesign for the original subsystems, Finally, the scheme is extended to the model reference tracking problem where global uniform boundedness of the tracking error to a compact set is established.