Wildfires have become increasingly frequent and intense worldwide over the past decades, raising concerns about their impacts on soil systems. However, limited information is available on how fire-induced heating affects the structural properties of clay-rich soil aggregates, particularly those dominated by iron and aluminum oxides. The objectives of this study were to: 1) assess the effects of simulated peak fire temperatures on the structural properties of clayey Oxisol aggregates; 2) investigate the relationship between aggregate stability (AS) and tensile strength (TS) under heating; and 3) identify the main biophysical mechanisms influencing AS and TS. Six clayey Oxisols were selected, and surface samples (0–5 cm; n = 6 per soil) were collected. Soil aggregates were classified into two size fractions (12–19 mm and 4–8 mm), and the clay fraction (< 0.002 mm) was isolated for complementary analyses. A laboratory setup was used to simulate a realistic fire temperature gradient ranging from 100 °C to 600 °C. The following properties related to aggregate structural strength were evaluated: soil organic matter (SOM) content, the oxalate/dithionite-extractable Fe ratio, mineralogical and micromorphological features, aggregate stability (AS), and tensile strength (TS). A distinct temperature threshold was identified at 300 °C, corresponding to a significant reduction in SOM and TS (R 2 = 0.99), accompanied by an increase in AS. Principal component analysis (PCA) and Pearson correlation results revealed a clear distinction between TS and other soil properties. The strong positive correlation between TS and SOM suggests that the decrease in organic matter caused by heating promotes the development of microcracks within aggregates, thereby reducing TS.