Knowledge about short-term assembly patterns and connectivity in estuarine mangrove ichthyofauna from the Indo-West Pacific region is scant. We assessed tidal and diel dynamics of intertidal and subtidal fish assemblages in the mangrove forest of Sikao Creek, southwest (Andaman coast) Thailand. Over three lunar cycles, two intertidal creeks were blocked at high water according to the interaction of tidal and diel cycles; the adjacent subtidal area was seined at low waters. A total of 18,033 individuals (93 species, 36 families) were caught. Catch mass and abundances were dominated by small-sized fishes (intertidal creeks: Apogon hyalosoma, Ambassis interrupta, Ambassis vachellii; subtidal area: A. vachellii, Neostethus lankesteri, Oryzias javanicus, Siganus javus). Of 45 intertidal species, 78 % were also caught in the subtidal, suggesting close connectivity between the two habitat types. Overall intertidal fish biomass (0.3 g m−2) and density (0.03 fish m−2) were relatively low. Specific combinations of tidal magnitude and diel cycle explained recurring changes in intertidal fish assemblage structures, clearly observed at the species level, in the number of species, diversity, evenness and biomass. This pattern was strongly driven by new moon-night abundance maxima of dominant A. vachellii, which also displayed fullest stomachs at this lunar-diel cycle interaction. We suggest that estuarine mangrove fish studies of the Andaman Sea region may suffer significant bias when the importance of tidal-lunar and diel cycle interactions are not considered because the role of Ambassidae will be strongly underestimated.