Recent　advances　in　cancer　research　have　highlighted　the　pivotal　role　of　tertiary　lymphoid　structures　(TLSs)　in　modulating　immune　responses,　particularly　in　breast　cancer　(BRCA).　Here,　we　performed　an　integrated　analysis　of　bulk　transcriptome　data　from　over　6000　BRCA　samples　using　biological　network-based　computational　strategies　and　machine　learning　(ML)　methods,　and　identified　LGALS2　as　a　key　marker　within　TLSs.　Single-cell　sequencing　and　spatial　transcriptomics　uncover　the　role　of　LGALS2　in　TLS-associated　dendritic　cells　(DCs)　stimulation　and　reveal　the　complexity　of　the　tumor　microenvironment　(TME)　at　both　the　macro　and　micro　levels.　Elevated　LGALS2　expression　correlates　with　prolonged　survival,　which　is　associated　with　a　robust　immune　response　marked　by　diverse　immune　cell　infiltration　and　active　anti-tumor　pathways　leading　to　a　‘hot’　tumor　microenvironment.　The　colocalization　of　LGALS2　with　TLS-associated　DCs　and　its　role　in　immune　activation　in　BRCA　were　confirmed　by　hematoxylin-eosin　(HE),　immunohistochemistry　(IHC),　and　in　vivo　validation　analyses.　The　identification　of　LGALS2　as　a　key　factor　in　BRCA　not　only　highlights　its　therapeutic　potential　in　novel　TLS-directed　immunotherapy　but　also　opens　new　avenues　in　patient　stratification　and　treatment　selection,　ultimately　improving　clinical　management.　©　The　Author(s)　2024.