A　novel　artificial　boundary　method　called　the　scaled　boundary　perfectly　matched　layer　(SBPML)　is　proposed　for　the　transient　analysis　of　elastic　wave　propagation　in　unbounded　domains.　This　method　constructs　a　generalized　perfectly　matched　layer　(PML)　around　the　finite　interior　subdomain　based　on　a　coordinate　transformation　inspired　by　the　scaled　boundary　finite　element　method　(SBFEM).　It　can　model　the　infinite　exterior　subdomains　of　heterogeneous　materials,　featuring　both　parallel　and　radial　straight-line　physical　surfaces　and　interfaces.　In　addition,　generally-shaped　(even　nonconvex-shape)　artificial　boundaries　can　be　used　to　provide　high　flexibility　in　choosing　the　geometry　of　the　finite　interior　subdomain.　Two　coordinate　mapping　technologies,　namely　the　modified　scaled　boundary　coordinate　transformation　from　the　SBFEM　and　the　complex　coordinate　stretching　from　the　PML,　are　successively　utilized　to　construct　the　proposed　method.　By　introducing　the　integral　of　stress　as　an　auxiliary　variable,　the　resulting　SBPML　subdomain　is　represented　in　the　time　domain　using　a　mixed　displacement-stress　unsplit-field　formulation.　This　has　the　added　benefit　of　allowing　for　easy　incorporation　into　standard　dynamic　finite　element　methods.　The　performance　of　the　SBPML　is　demonstrated　through　several　numerical　examples,　including　P-SV　and　SH　wave　propagation　problems　occurring　in　unbounded　domains　with　complex　geometries　and　heterogeneous　material　properties.　The　numerical　results　are　evaluated　in　comparison　to　both　reference　solution　and　existing　artificial　boundary　methods,　ultimately　demonstrating　the　flexibility,　robustness,　accuracy,　and　stability　of　the　proposed　approach.