An analytical protocol was developed to identify arsenic (As) in soil samples and in the ≤10 μm fraction of surface dust samples (fine surface dust-FSD). Single-particle identification of trace As constituents was undertaken by combining scanning electron microscopy with automated image analyses and high-resolution, transmission electron microscopy. Two forms of As association with iron and aluminum nanoscale phases were identified. In the predominant one, As was identified in oriented aggregates formed by crystalline nanoparticles of Fe-(hydr)oxides. In the FSD samples, As was additionally detected in an assembly of hematite and goethite nanocrystals forming larger particles of few hundreds of nanometers, often entangled with phyllosilicates. These mixed phases carried various elements, such as P, Ba, Pb, among others. Even rare As-bearing phases (e.g., l to 9 particles out of approx. 30,000 particles analyzed), such as arsenopyrite and ferric arsenate, and possibly scorodite were identified in some samples. The developed analytical protocol brings a novel and practical contribution to As speciation in environmental samples.