Microplastic (MP) concentrations were assessed in a 210Pb-dated sediment core from Ciénaga Los Vásquez, a coastal lagoon in Cartagena, Colombian Caribbean. The concentration of MPs increased over time reaching up to 5358 items/kg d.w. in the year 2021 ± 0.31, which is consistent with several studies around the world reflecting the increase in plastic production in recent decades, consumption, and lack of management that influence pollution in coastal settings. Nile Red-induced fluorescence under 365 nm light and automatic counting with ImageJ allowed for a cheap and time-effective assessment of MPs and the detection of particles down to 44.27 μm. Most MPs were small (95% < 299 μm), representing a commonly overlooked size range by conventional methodologies. These results constitute the first report along Colombian coasts of MP concentrations over time using a sediment core.