Plant invasions modify habitat structure and resource availability, potentially altering the assembly of belowground fauna communities. Little is known about how soil microarthropod communities respond across different stages of plant invasion. This study investigated whether invasion density and estimated stand age influence soil microarthropod community composition and abundance along an Ailanthus altissima invasion gradient.Thirty invaded urban forest stands were selected in Montpellier (France) and classified into three invasion-density classes (low: 0.06–1.69, intermediate: 1.72–4.00 and high: 4.24–9.04 individuals m⁻²; n = 10 per class) and three invasion-age classes estimated from the maximum stem circumference of A. altissima (young <10 cm; intermediate 10–30 cm; old >30 cm; n = 10 per class). Invasion density was quantified as the combined density of mature trees, intermediate individuals and root suckers. At each stand, four soil cores were collected, and soil microarthropods were extracted using the McFadyen method and identified to the lowest possible taxonomic level.Invasion age did not significantly affect either soil microarthropod community composition or the abundance of the main taxonomic groups. In contrast, invasion density was associated with marked quantitative changes. High-density A.s altissima stands supported significantly lower abundances (P < 0.05) of total Acari, non-oribatid mites, total Collembola, Entomobryomorpha and Poduromorpha groups than both low- and intermediate-density stands, whereas the latter two invasion classes showed comparable abundances. Community composition remained relatively stable across the invasion gradient, although older stands tended to exhibit lower multivariate dispersion, suggesting a reduction in between-site community heterogeneity with increasing stand age.These findings indicate that soil microarthropod communities respond non-linearly to A. altissima invasion. Increasing invasion density was associated with taxon-specific quantitative responses, with several dominant groups remaining abundant at low and intermediate invasion densities but declining once A. altissima reached high levels of dominance.