Multiverse analysis is increasingly recognised as a systematic framework for assessing the robustness of scientific results across alternative defensible data processing and analysis pipelines. However, defining the multiverse, by identifying defensible combinations of options across multiple nodes in the analysis workflow, remains a cognitively and logistically demanding and complex task. Consequently, documentation of how multiverse analyses are constructed and the rationale behind decisions made is often incomplete, which risks the transparency and interpretability of robustness claims. The Systematic Multiverse Analysis Registration Tool (SMART) addresses this gap. SMART guides users from diverse scientific disciplines through the construction of multiverse analyses via a transparent, stepwise workflow. It guides the users through all defensibility and equivalence decisions, and documents each decision made. This supports the creation of complete multiverses within user-defined criteria, with visual and numerical feedback to highlight potential errors along the procedure. The exportable documentation can be used for preregistration or included as supplementary material alongside the published manuscript to report uncertainty at the level of multiverse construction. By increasing the transparency, reproducibility, and rigour in multiverse construction, and bringing cohesion to this procedure across multiverse analyses, SMART facilitates more interpretable robustness assessments and contributes to the broader goals of open and reproducible science.