Satheesh, K.A.Johnson, P.Kamaraj, K.2026-02-032025Indian Journal of Pure and Applied Mathematics, 2025, 56, 3, pp. 1026-1035195588https://doi.org/10.1007/s13226-025-00818-1https://idr.nitk.ac.in/handle/123456789/20106We present some results to characterize the reverse order law for Moore-Penrose inverse of closed densely defined operators on Hilbert spaces. We use the basic properties of the Moore-Penrose inverse of closed operators to prove our results. We provide an example to show that the reverse order law for Moore-Penrose inverse of unbounded closed densely defined operators may not hold good in general. We also provide a method to find the Moore-Penrose inverse of a closed densely defined operator as an application of the reverse order law using polar decomposition. © The Indian National Science Academy 2025.Closed densely defined operatorMoore-Penrose inversePolar decompositionReverse order law