Pore‐water and solid‐phase analysis was undertaken to study the effects of different environmental conditions on redox processes in mangrove soils on the southern coast of Brazil. Three Rhizophora mangle(L.) mangrove forests were studied in different physiographic positions—riverine, fringe, and basin forest—and substrate compositions. Our results indicate different geochemical conditions in soils from different physiographic positions. In the fringe soils, the low acid‐volatile sulfide and pyrite Fe contents, associated with high values of Fe2+in pore water, indicate oxic and suboxic conditions in surface layers (0–10 cm). Below this depth, anoxic conditions prevail and sulfate reduction becomes the dominant process of organic matter decay, as indicated by higher concentrations of dissolved HS−and pyrite Fe. In the basin forest, long periods of tidal flooding and low rates of litter export maintained anoxic conditions and high organic matter contents, necessary for anoxic respiration to take place, as shown by the low Fe2+and HS−concentrations in pore water, along with high degree of Fe pyritization values and high concentrations of pyrite Fe. In the riverine soils, the lowest concentrations of pyrite Fe and the highest concentrations of HS−in pore water were obtained in response to low availability of reactive Fe (0–10cm, 31.8 ± 8.0 mmol kg−1; 20–30 cm, 43.2 ± 32.3 mmol kg−1) due to its sandy texture, which limits pyrite synthesis. Our results indicate that the participation and intensity of each process may vary within different physiographic positions because of the effects on the frequency and duration of tidal flooding and soil composition. Our results also show that R. mangleis present on substrates with different geochemical conditions (oxic, suboxic, and anoxic).