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Abstract Relative relocation techniques are widely used to improve the resolution of earthquake hypocenter positions. Here, we present GrowClust3D.jl—an open-source software package written in the programming language Julia that builds and improves upon the original GrowClust algorithm, an established relative relocation technique based on cluster analysis instead of a more traditional matrix inversion approach. The adoption of Julia’s modern programming environment allows for greater flexibility in GrowClust3D.jl’s algorithm design and its computational implementation. Notable additions to the GrowClust3D.jl package include (1) several parallel processing options to improve efficiency in uncertainty quantification routines, (2) incorporation of geographic map projections and station elevations during the relocation process, and (3) the ability to use travel-time tables derived from 3D velocity models. We demonstrate the new features of the software package on relocation problems of different scales in Nevada, California, Texas, and New Zealand, where in the latter two cases the use of a 3D velocity model helps resolve structures that remain obscure with earlier versions of GrowClust. We expect that the new GrowClust3D.jl software package will become a valuable public resource for the earthquake science community.