Seagrass meadows around the Andaman Sea are globally significant, but declining rapidly. Assessment of the existing seagrass population dynamics is essential to facilitate effective conservation measures. We studied population dynamics of the seagrass Thalassia hemprichii at 3 locations in the Andaman and Nicobar Islands in the Andaman Sea. At each location, 2 sites were assessed, one with mangroves (MG) and another without mangroves (WMG). Quadrat and corer sampling techniques were used to collect seagrass and sediment samples. Reconstruction techniques were used to derive population dynamics of T. hemprichii. Sand fractions dominated (>90%) the T. hemprichii meadows, with silt comprising a higher percentage only at the MG sites. The density, biomass, productivity and horizontal meadow migration of T. hemprichii were higher at the MG sites. The number of leaves shoot^-1, vertical rhizome (VR), VR internode length, number of VRs shoot^-1 and vertical growth were higher at the WMG sites. T. hemprichii required less time to produce a single leaf at the MG sites than at the WMG sites. Plants associated with mangroves had 4 to 5 yr of longevity, with higher numbers of younger plants. Population growth rates were positive at all sites except at the WMG site of Burmanallah. Our results provide evidence that mangrove ecosystems have a positive impact on seagrass meadow traits and population dynamics. Therefore, it is crucial to focus on the ecological connectivity between seagrasses and associated coastal ecosystems, as it is pivotal to increase our understanding of the important link between coastal ecosystems and ecosystem functioning.