Let $G$ be a locally compact group and $K$ a compact subgroup such that the algebra $L^1(G{)}^{♯}$ of biinvariant integrable functions is commutative. We characterize the $G$-invariant Dirichlet forms on the homogeneous space $G/K$ using harmonic analysis of $L^1(G{)}^{♯}$. This extends results from Ch. Berg, Séminaire Brelot-Choquet-Deny, Paris, 13e année 1969/70 and J. Deny, Potential theory (C.I.M.E., I ciclo, Stresa), Ed. Cremonese, Rome, 1970. Every non-zero $G$-invariant Dirichlet form on a symmetric space $G/K$ of non compact type...