In mathematics, **Radó's theorem** is a result about harmonic functions, named after Tibor Radó. Informally, it says that any "nice looking" shape without holes can be smoothly deformed into a disk.

Suppose Ω is an open, connected and convex subset of the Euclidean space **R**^{2} with smooth boundary ∂Ω and suppose that **D** is the unit disk. Then, given any homeomorphism μ : ∂ **D** → ∂ Ω, there exists a unique harmonic function *u* : **D** → Ω such that *u* = μ on ∂**D** and *u* is a diffeomorphism.