Microplastics (<5 mm) are ubiquitous in the marine environment, occurring in both sediments and surface waters worldwide. However, few studies have documented the presence of microplastics and tire wear particles in coastal rivers. A survey of microplastics and low-density tire wear particles (≥63 μm) in the sediment and surface water of the three major tributaries within the Charleston Harbor estuary was conducted. Intertidal sediment, subtidal sediment, and sea surface microlayer concentrations ranged from 0 to 652 microplastics/m2, 3–4,375 microplastics/kg wet weight, and 3–36 microplastics/L, respectively. Blue fibers and tire wear particles were the two most abundant microplastic types observed, constituting 26.2% and 17.1%, respectively, of total microplastics. Tire wear particles were primarily identified by morphology, and ATR-FTIR analysis was conducted for a small subset (n = 5) of larger particles (≥500 μm). The present study provides the first microplastic field assessment of low-density tire wear particles in estuarine tributaries. Display omitted •Microplastics were widely distributed in the water and sediments of each river.•Blue fibers and tire wear particles were the two most abundant microplastic types.•Morphological and analytical evidence of tire wear particles in the environment