Xinjiang, northwestern China, encompasses sharply contrasting desert–wetland and montane forest ecosystems, providing a natural setting to study species limits and habitat-associated morphological variation in Agaricus subgen. Pseudochitonia. We examined 303 specimens collected from 2021 to 2025 and generated 905 sequences across three loci (ITS, LSU, tef1). Species delimitation followed an integrative framework incorporating morphology, multilocus phylogeny, pairwise genetic distances, chemical reactions, and habitat data. We recognized ten species from Xinjiang. Among them, Agaricus acanthosquamosus is described as a new species in sect. Bohusia. It is recovered as a sister to A. bohusii and is distinguished by conspicuous squarrose spinose pileus scales, subglobose to broadly ellipsoid basidiospores, a solitary to scattered fruiting habit, and reddening context. We also document newly observed morphological variation in several previously described species. These include irregularly clavate cheilocystidia in A. desjardinii, clavate pleurocystidia in A. subperonatus, non-digitate pleurocystidial apices in A. sinodeliciosus, and habitat-associated macromorphological variation in A. xanthodermus. These observations extend the known morphological range of each taxon. Exploratory PCA, PERMANOVA, and trait comparisons revealed significant assemblage-level morphological structuring between the Ebinur Lake desert–wetland assemblage and the Qiongkushitai montane forest assemblage, notably in basidioma size, robustness, and basidiospore dimensions. In addition, we compiled a comprehensive identification key and species checklist for Agaricus subgen. Pseudochitonia in Xinjiang. These integrative results highlight both the diversity of subgen. Pseudochitonia in arid northwestern China and the value of combining morphological, ecological, and molecular evidence for accurate species delimitation.