We establish a large deviation principle for the Stratonovich stochastic nonlinear heat equation posed on a smooth bounded domain \(\mathscr {O} \subset \mathbb {R}^{d}\) with Dirichlet boundary conditions. The dynamics evolve on the Hilbert manifold \(\mathscr {M} = \{u \in L^{2}(\mathscr {O}): |u|_{{L}^{2}} = 1\}\) and are driven by multiplicative noise that is tangent to \(\mathscr {M}\) . Adopting the Budhiraja–Dupuis weak convergence framework, we verify the required compactness and stability conditions for the associated controlled skeleton system, and derive the corresponding good rate function in the space \(X_{T} = C([0,T];\textrm{V}) \cap {L}^{2}(0,T;\textrm{E})\) .