The stress regime in the Illinois Basin was investigated to assess how the rock column might respond to the injection of fluids, including coproduced formation brines and supercritical CO₂.This response is a concern because injection practices could increase pore fluid pressure and potentially induce seismicity. Data were collected to determine the magnitude and orientation of a three-component stress field: vertical stress (S_v) and minimum (S_h) and maximum (S_H) horizontal stresses. The S_v was evaluated with a six-layer lithostratigraphic column. A two-layer pressure–depth S_v model was generated for the central part of the basin, and a single pressure gradient model was constructed for the surrounding region. In the central part of the basin, the S_v gradient is 24.9 MPa/km (1.11 psi/ft) to a depth of 2134 m (7000 ft), followed by a gradient of 27.1 MPa/km (1.20 psi/ft) below 2134 m (7000 ft). For the area surrounding the deep basin, the S_v gradient was 25.5 MPa/km (1.13 psi/ft). The S_h was evaluated from multiple data sources, primarily hydraulic fracture records or extended leak-off tests. The S_h gradient calculations ranged from 24.1 to 27.3 MPa/km (1.07 to 1.21 psi/ft). The S_h values for the basal Paleozoic clastic units are lower than those for units in the overlying horizons. The S_H was based on a critically stressed model yielding values between 40.0 and 82.6 MPa/km (1.77 to 3.65 psi/ft). Stress orientation data for the Illinois Basin were collected from multiple sources. The orientation of S_H across the study area is relatively uniform in strike at approximately N60°E. Marked deviations in S_H result from localized structural discontinuities.