Metastatic　relapse　is　a　leading　cause　of　cancer-associated　death　and　one　of　the　major　obstacles　for　effective　therapy　against　triple-negative　breast　cancer.　To　address　this　problem,　a　miRNA-delivering　nanocapsule　technology　based　on　hyaluronic　acid　(HA)/protamine　sulfate　(PS)　interpolyelectrolyte　complexes　(HP-IPECs)　is　developed　for　efficient　encapsulation　and　intracellular　delivery　microRNA-34a　(miR-34a),　which　is　a　potent　endogenous　tumor　suppressor　of　breast　cancer.　The　nanocapsules　are　successfully　generated　through　a　self-assembly　approach　mediated　by　an　electrostatic　interaction.　In　vitro　and　in　vivo　experiments　illustrate　that　miR-34a　can　be　efficiently　encapsulated　into　HP-IPECs　and　delivered　into　breast　cancer　cells　or　breast　cancer　tissues.　Nanocomplex-assisted　delivery　of　miR-34a　induces　cell　apoptosis　and　suppresses　migration,　proliferation,　and　tumor　growth　of　breast　cancer　cells　via　targeting　CD44　and　a　Notch-1-signaling　pathway.　The　obtained　results　suggest　that　HP-IPECs　have　a　great　potential　as　a　biodegradable　vector　for　microRNA-based　therapy　against　triple-negative　breast　cancer.