Oil spillage is one of the common pollution events of global water-soil ecosystems. A comprehensive investigation on heavy metals pollution of surface water and sediments was conducted after oil spill incident in Sela River and its tributaries of the Sundarbans mangrove forest ecosystems, Bangladesh. Water and sediment samples were collected from the preselected sampling points in Sela River, and the elemental (Pb, Cd, Cr, Co, Cu, Ni, Fe, As, Hg, Mn, Zn, Ca, Mg, Na, and K) analysis was done using atomic absorption spectrometer (AAS). This study revealed that the descending order for the average concentration of the studied elements were found to be Mg > Co > Na > Ni > K > Ca > Pb > Fe > Mn > Cr > Cd > Zn > Cu respectively, while As and Hg in water samples were found to be below detection limit (BDL). However, some of the toxic elements in the Sela River water samples were exceeded the permissible limit set by the World Health Organization (WHO) with a descending order of Co > Cd > Pb > Ni respectively. Based on the water quality index (WQI), metal pollution index (MPI), and metal quality index (MI), the Sela River water is not suitable for drinking but may be used for irrigating agricultural and vegetable crops. On the other hand, elemental concentration in the sediment samples were found to be the following descending order of Fe > Mg > Na > K > Ca > Mn > Zn > Cr > Cu > Pb > As > Cd respectively. Several pollution assessment indices: contamination factor (Cf), degree of contamination (Cd), modified degree of contamination (mCd), pollution load (PLI), enrichment factor (EF), geo-accumulation (Igeo) indices were followed to assess the sediment systems pollution in the study area. Considering sediment quality indices, this study revealed that the river sediment had higher contamination factor (Cf) values for Cd, moderate values for Pb, Cr, Cu, Zn, Mg, and As, and low values for Mn, Fe, Ca, Na, and K. Among the studied heavy metals, Cd content was highest in both water and sediment samples, which confirming that Cd, insoluble or suspended form, was more likely to be strongly deposited and bound in sediments from water. Principal component and correlation analyses suggested that the sources of heavy metals pollution were mainly anthropogenic along with the geogenic sources in the study area.