This　paper　presents　an　efficient　monolithic　computational　homogenization　model　for　transient　nonlinear　hydro-mechanical　analysis　within　the　framework　of　Numerical　Manifold　Method　(NMM).　The　proposed　model　is　on　the　same　theoretical　basis　as　the　FE2　method.　The　scale　transitions　are　achieved　through　the　extended　Hill-Mandel　theorem　so　that　the　microscopic　fluid　and　solid　dynamic　effects　are　fully　incorporated.　The　two-scale　simulations　are　solved　in　a　monolithic　manner　and　the　microscopic　problems　of　all　macroscopic　integration　points　are　decoupled　from　each　other　to　prevent　size　of　the　system　of　equations　from　soaring　to　exceedingly　large.　By　conveying　microscale　unbalanced　forces　and　tangent　operators　to　the　macroscale　level,　the　micro-　and　macroscale　problems　are　solved　in　the　same　Newton　loop　such　that　unnecessary　microscopic　iterations　based　on　estimated　macroscopic　variables　in　the　conventional　nested　homogenization　m　are　avoided.　By　solving　benchmark　numerical　examples,　the　proposed　model　proves　to　be　capable　of　capturing　transient　hydro-mechanical　responses　accurately.　Moreover,　in　contrast　to　the　conventional　nested　homogenization　model,　the　proposed　model　saves　around　40%　of　computational　costs　for　nonlinear　hydro-mechanical　analysis.　Using　the　framework　of　numerical　manifold,　the　presented　model　can　be　easily　extended　to　multiscale　analyses　involving　complex　boundaries,　interfaces　and　fractures.　©　2023