<p><strong>Abstract.</strong> Dissolved iron (dFe) is essential for biogeochemical reactions in oceans, such as photosynthesis, respiration and nitrogen fixation. Currently, large uncertainties remain on riverine dFe inputs, especially for tropical rivers in Southeast Asia. In the present study, dFe concentrations and distribution along the salinity gradient in the Rajang River in Malaysia, and three blackwater rivers draining from peatlands, including the Maludam River, the Sebuyau River, and the Simunjan River, were determined. In the Rajang River, the concentration of dFe in fresh water (salinity&#8201;<&#8201;1) in the wet season (March 2017) was higher than that in the dry season (Auguest 2016), which might be related to the resuspension of sediment particles and soil erosions from cropland in the watershed. In the Rajang Estuary, an intensive removal of dFe in low salinity waters (salinity&#8201;<&#8201;15) was observed, likely due to the salt-induced flocculation and the absorption onto suspended particulate matters (SPM). However, dFe concentration enhancements in the wet season occured in some sampling sites, which may be related to the desorption from SPM and agriculture activities. On the other hand, dFe was conservatively distributed in high salinity waters (salinity&#8201;>&#8201;15), which may result from the association between dFe and pelagic organic matters. In the blackwater rivers, concentrations of dFe reached 44.2&#8201;&#956;mol&#8201;L<sup>&#8722;1</sup>, indicating a great contribution from peatland. The dFe flux derived from the Rajang Estuary to the South China Sea was (6.4&#8201;&#177;&#8201;2.3)&#8201;&#215;&#8201;10<sup>5</sup>&#8201;kg&#8201;yr<sup>&#8722;1</sup>. For the blackwater river, the dFe flux was approximately (1.1&#8201;&#177;&#8201;0.5)&#8201;&#215;&#8201;10<sup>5</sup>&#8201;kg&#8201;yr<sup>&#8722;1</sup>, in the Maludam River. The anthropogenic activities may play an important role in the dFe yield, such as the Serendeng tributary of the Rajang River, and Simunjan River, where intensive oil palm plantations were observed.</p>