The root elongation rate represents a biophysical process that can be directly affected by mechanical, water, thermal, and gaseous stresses in the soil to be used as a soil physical quality indicator. The objective of this study was to determine sugarcane root growth parameters under soil physical stress for different root diameter classes in an Oxisol from the Southeast of Brazil. The experimental design was entirely randomized in a factorial scheme 5 × 2 (mechanical × water stress) with three replications. The factor mechanical stress was composed of five compaction levels (1.04; 1.12; 1.19; 1.28; 1.36 Mg m–3). The factor water stress was composed of two matric potentials (–6 kPa and –33 kPa). Soil samples were collected from the 0.0–0.2 m layer of an Oxisol with a clayey texture. Pre-sprouted sugarcane seedlings were transplanted and conditioned in a growth chamber. Root length, volume, surface area, and diameter were quantified to generate root growth models as a function of physical stresses in the soil. Soil penetration resistance increases from 1.4 to 5 MPa reduced root elongation rate from 3.5 to 1.35 cm day–1 (–59%) and the average number of roots from 11 to 6 segments (–45%), respectively. The root volume, surface area, and length were reduced because of the increase in the compaction level. Coarse root diameter (1–2 mm) was weakly impacted by mechanical stress, whereas fine root diameter (0.5–1 mm) was more growth limited in compacted soils. The root elongation rate of sugarcane was modelled as a function of mechanical and water stress. Mechanical stress mainly affects the growth of sugarcane roots with small diameter.