We study the boundary extraordinary transition of a three-dimensional (3D) tricritical O(N) O ( N ) model. We first compute the mean-field Green’s function with a general coupling of |\vec \phi|^{2n} | ϕ → | 2 n (with n=3 n = 3 corresponding to the tricritical model) at the extraordinary phase transition. Then, using layer susceptibility, we obtain the boundary operator expansion for the transverse and longitudinal modes within the \epsilon=3 - d ϵ = 3 − d expansion. Based on these results, we demonstrate that the tricritical point exhibits an extraordinary transition characterized by an ordered boundary for any N N . This provides the first nontrivial example of continuous symmetry breaking in 2D in the context of boundary criticality.