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Abstract A unified fluid theory of ionospheric electrostatic instabilities is presented that includes thermal effects due to nonisothermal processes for arbitrary ion magnetization, background density gradient, and wave propagation. The theory considers arbitrary altitude within the limits imposed by the fluid and collisional models and integrates the ion‐thermal instability (ITI) with the Farley‐Buneman and gradient‐drift plasma instabilities (FBI and GDI). A general dispersion relation is obtained and solved numerically for the complex wave frequencyωby using either an iterative or a polynomial (quadric) form inω. An analytic explicit expression for the instability growth rate is also derived under the local and slow growth approximations. The previously considered limiting cases of the FBI/ITI at long wavelengths and the FBI/GDI for isothermal plasma are successfully recovered. In the high‐latitudeE‐region near 110 km in altitude, thermal effects are found to be destabilizing at long wavelengths near  m and stabilizing at shorter wavelengths near 10 m. In theF‐region, the effects are destabilizing at  m but much weaker that those of GDI for moderate gradients. At shorter wavelengths, they become comparable so that a significant fraction of propagation directions at  m have positive growth rates, in contrast with the isothermal FBI/GDI case, where stronger gradients are needed to destabilize the plasma at these short wavelengths. The overall conclusion is that the thermal effects modify the growth rate terms traditionally associated with FBI and GDI rather than being purely additive.