In earlier work we identified a global, non-axisymmetric instability associated with the presence of an extreme in the radial profile of the key function L(r) ≡ (ΣΩ/κ^2) S^2/Γ in a thin, inviscid, nonmagnetized accretion disk. Here, Σ(r) is the surface mass density of the disk, Ω(r) the angular rotation rate, S(r) the specific entropy, Γ the adiabatic index, and κ(r) the radial epicyclic frequency. The dispersion relation of the instability was shown to be similar to that of Rossby waves in

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... etary atmospheres. In this paper, we present the detailed linear theory of this Rossby wave instability and show that it exists for a wider range of conditions, specifically, for the case where there is a "jump" over some range of r in Σ(r) or in the pressure P(r). We elucidate the physical mechanism of this instability and its dependence on various parameters, including the magnitude of the "bump" or "jump," the azimuthal mode number, and the sound speed in the disk. We find large parameter range where the disk is stable to axisymmetric perturbations, but unstable to the non-axisymmetric Rossby waves. We find that growth rates of the Rossby wave instability can be high, ∼ 0.2 Ω_ K for relative small "jumps" or "bumps". We discuss possible conditions which can lead to this instability and the consequences of the instability.