We prove a necessary and sufficient condition for the graded algebra of automorphic forms on a symmetric domain of type IV to be free. From the necessary condition, we derive a classification result. Let M be an even lattice of signature (2, n) splitting two hyperbolic planes. Suppose Γ is a subgroup of the integral orthogonal group of M containing the discriminant kernel. It is proved that there are exactly 26 groups Γ such that the space of modular forms for Γ is a free algebra. Using the sufficient condition, we recover some well-known results.