The inﬁnite-dimensional unitary group U(∞) is the inductive limit of growing compact unitary groups U(N ). In this paper we solve a problem of harmonic analysis on U(∞) stated in [Ol3]. The problem consists in computing spectral decomposition for a remarkable 4-parameter family of characters of U(∞). These characters generate representations which should be viewed as analogs of nonexisting regular representation of U(∞). The spectral decomposition of a character of U(∞) is described by the spectral measure which lives on an inﬁnite-dimensional space Ω of indecomposable characters. .