The trilinear Higgs coupling $λ_{hhh}$ is crucial for determining the structure of the Higgs potential and for probing possible effects of physics beyond the standard model. Focusing on the two-Higgs-doublet model as a concrete example, we identify parameter regions in which $λ_{hhh}$ is significantly enhanced with respect to the standard model. Taking into account all relevant corrections up to the two-loop level, we show that already current experimental bounds on $λ_{hhh}$ rule out significant parts of the parameter space that would otherwise be unconstrained. We illustrate the interpretation of the results on $λ_{hhh}$ for a benchmark scenario. Similar results are expected for wide classes of models with extended Higgs sectors.