The　growth　of　Sn-rich　group-IV　semiconductors　at　the　nanoscale　can　enrich　the　understanding　of　the　fundamental　properties　of　metastable　GeSn　alloys.　Here,　we　demonstrate　the　effect　of　the　growth　conditions　on　the　morphology　and　composition　of　Ge/GeSn　core/shell　nanowires　by　correlating　the　experimental　observations　with　a　theoretical　interpretation　based　on　a　multiscale　approach.　We　show　that　the　cross-sectional　morphology　of　Ge/GeSn　core/shell　nanowires　changes　from　hexagonal　to　dodecagonal　upon　increasing　the　supply　of　the　Sn　precursor.　This　transformation　strongly　influences　the　Sn　distribution　as　a　higher　Sn　content　is　measured　under　the　{112}　growth　front.　Ab　initio　DFT　calculations　provide　an　atomic-scale　explanation　by　showing　that　Sn　incorporation　is　favored　at　the　{112}　surfaces,　where　the　Ge　bonds　are　tensile-strained.　A　phase-field　continuum　model　was　developed　to　reproduce　the　morphological　transformation　and　the　Sn　distribution　within　the　wire,　shedding　light　on　the　complex　growth　mechanism　and　unveiling　the　relation　between　segregation　and　faceting.　The　tunability　of　the　photoluminescence　emission　with　the　change　in　composition　and　morphology　of　the　GeSn　shell　highlights　the　potential　of　the　core/shell　nanowire　system　for　optoelectronic　devices　operating　at　mid-infrared　wavelengths.