Functionalized metal–organic frameworks (MOFs) are increasingly recognized as efficient materials for the selective removal of synthetic dyes from aqueous solutions. To enhance selectivity, MOFs were functionalized with both electron-donating and electron-withdrawing groups, specifically –NH₂, –COOH, and –SO₃H, thereby adjusting their surface characteristics to favor interactions with oppositely charged dye species. Methylene blue and Congo red were selected as model cationic and anionic dyes, respectively, and adsorption performance was evaluated across a range of pH values and temperatures. Key interaction forces such as electrostatic attraction, hydrogen bonding, π–π stacking, and pore accessibility were considered. Adsorption behavior was analyzed through kinetic and isotherm models, including pseudo-second-order and Langmuir/Freundlich equations, revealing that the functionalized MOFs demonstrated both high dye uptake and fast adsorption rates. This work examines how cationic and anionic dyes interact with chemically modified MOFs, with an emphasis on elucidating their adsorption mechanisms. Characterization techniques including zeta potential, FTIR spectroscopy, UV-Visible spectroscopy and pH-responsive adsorption studies confirmed that adsorption mechanisms were influenced by both the dye charge and the functional groups present on the MOFs. It was observed that amino-functionalized MOFs were more effective in capturing anionic dyes, whereas sulfonated frameworks showed enhanced binding with cationic dyes. These results underscore the role of precise functionalization in tailoring the adsorption properties of MOFs. The mechanistic insights gained from this study can inform the design of next-generation MOF-based materials aimed at efficient and selective dye removal in wastewater treatment applications.