In northern Peru, the mangrove crab Ucides occidentalis is of great importance due to its ecological, economic, and social role. In this study, we reported for the first time the presence of microplastics in the gills and digestive tract of the mangrove crab U. occidentalis derived from local markets in Tumbes. Microplastics were identified in 100% of the crabs analyzed with a total of 921 items, 475 items (52.57%) found in the gills, and 446 (48.43%) found in the digestive tract. The size range was established in 2 to 250 µm, 250 to 500 µm, 500 to 1 mm, and 1 to 5 mm, microplastics with sizes between 2 and 250 µm were the most common with 53.79% in the digestive tract and 90% in the gills. A total of six different types of microplastic were recorded; The highest percentages for each tissue were fibers (59.64%–61.05%) and films (19.28%−36.63%), with clear fibers being the most prevalent microplastic type in both tissues. Microplastics with less than 250 µm size were found 90% in the gills and 53.79% in the crab digestive tract. Although the present study is a baseline for rapid identification of microplastics in mangrove crab, we suggested that these findings provided more information on the state of contamination as well as food security alert for local markets.