We compare three sequential sampling models, the Race model, the leaky competing accumulator model (LCA) and the drift diffusion model (DDM), as novel computational accounts of choices and response times in semantic relatedness decisions. We focus on two empirical benchmarks, the relatedness effect, denoting faster ”related” than ”unrelated” decisions when judging the relatedness of word pairs, and an inverted-U shaped relationship between response time and the relatedness strength of word pairs. Using simulations, we show that the LCA and DDM, but not the Race model, can reproduce both effects. Furthermore, we show that the LCA and DDM differ in their account of the relatedness effect, producing it under different circumstances and using different mechanisms. This observation offers a critical test, involving a novel, inverted relatedness effect for low-relatedness word pairs. Reanalyzing a publicly available data set, we obtained credible evidence of such an inverted relatedness effect, which is consistent with the LCA, but not the DDM. These results provide strong support for the LCA as an accurate computational account of semantic relatedness decisions and suggest an important role for decision-related processes in (semantic) memory tasks.