Heavy metal (HM) contamination in soil and plants endangers living organisms due to its toxic, persistent, and bioaccumulative nature. In this study, soil and grass samples were collected from the Buriganga, Turag, and Bongshi riverside areas of megacity Dhaka, Bangladesh. In this investigation, pollution levels, ecological risk, and HM transfer from soil to grass were assessed using the geo-accumulation index, contamination factor, ecological risk index, and correlation analysis. HM concentrations in both soil and grass were found in descending order of Lead (Pb) > Chromium (Cr) > Nickel (Ni) > Copper (Cu) > Cadmium (Cd). The highest concentrations of Pb were 73.45 ± 15.76 mg/kg and 64.23 ± 8.68 mg/kg in soil and grass, respectively. The maximum geo-accumulation index was < 1, indicating the unpolluted soil quality. The mean contamination factors of HMs were 0.85 in Cr (low contamination), 0.96 in Cu (low contamination), 3.61 in Ni (considerable contamination), 0.039 in Cd (low contamination), and 2.29 in Pb (moderate contamination). The degree of contamination was above 6, indicating moderate contamination. One site near the Bongshi River and six sites in the dairy farm area showed a pollution load index value (PLI) < 1, indicating no pollution. The other 17 areas showed PLI > 1, indicating polluted sites. Ecological risk index analysis indicates that all the sites are ecologically safe, but HM mobility from soil to grass indicates a high accumulation of HM in grass. Effective environmental management is essential to control HM pollution at its source and protect riverside ecosystems.