Folding of RNA is subject to a competition between entropy, relevant at high temperatures, and the random, or random looking, sequence, determining the lowtemperature phase. It is known from numerical simulations that for random as well as biological sequences, high-and low-temperature phases are different, e.g. the exponent ρ describing the pairing probability between two bases is ρ = 3 2 in the high-temperature phase, and ρ ≈ 4 3 in the low-temperature (glass) phase. Here, we present, for random sequences, a field theory of the phase transition separating highand low-temperature phases. We establish the existence of the latter by showing that the underlying theory is renormalizable to all orders in perturbation theory. We test this result via an explicit 2-loop calculation, which yields ρ ≈ 1.36 at the transition, as well as diverse other critical exponents, including the response to an applied external force (denaturation transition).