The　rich　availability　of　seawater　and　sea　sand　offers　an　alternative　material　resource　for　concrete　production.　However,　the　mechanical　performance　and　durability　of　such　concrete　might　not　satisfy　the　construction　requirements.　Due　to　this,　this　study　has　first　investigated　the　effects　of　varying　dosages　of　ultrafine　metakaolin　(UMK)　and　nano-TiO2　(NT)　as　the　supplementary　cementitious　materials　on　the　mechanical　properties　of　concrete　produced　using　natural,　untreated　seawater　and　sea　sand.　The　workability,　compressive　strengths,　elastic　moduli　and　flexural　strengths　have　been　explored　for　the　concrete　using　the　unary　(ordinary　Portland　cement,　OPC),　binary　(OPC　and　UMK)　and　ternary　(OPC,　UMK　and　NT)　mixtures.　The　experimental　results　indicated　the　significant　enhancement　in　the　mechanical　properties　of　the　modified　concrete.　The　cube　compressive　strength,　axial　compressive　strength,　the　splitting　tensile　strength,　the　elastic　modulus　and　the　flexural　strength　have　increased　by　22.29　%,　22.82　%,　9.76　%,　16.02　%　and　44.44　%,　respectively.　After　that,　the　microstructural　aspects　expressed　by　SEM　and　XRD　were　also　investigated　for　revealing　the　contributions　of　the　NT　and　the　UMK　to　the　macroscopic　mechanical　performance　of　the　seawater　sea-sand　concrete.　The　SEM　analysis　revealed　a　reduction　in　porosity　and　improved　interfacial　zones　in　the　concrete　containing　the　UMK　and　NT.　The　XRD　analysis　confirmed　that　the　addition　of　UMK　and　NT　promoted　the　calcium　silicate　hydrate　(C-S-H)　gel　formation,　mitigating　the　alkali-aggregate　reactions.　It　was　found　that　the　addition　of　UMK　and　NT　could　improve　the　microstructure　of　the　seawater　sea-sand　concrete,　thereby　enhancing　the　mechanical　properties.　Subsequently,　the　corrosion　test　conducted　in　a　natural　marine　tidal　environment　revealed　that,　after　the　360　tidal　corrosion　cycles,　only　the　ternary　mixed　concrete　maintained　its　structural　integrity　without　the　strength　degradation,　highlighting　its　superior　durability　in　a　marine　condition.　Lastly,　analysis　of　variance　was　also　performed　to　statistically　evaluate　the　effects　of　UMK　and　NT　as　the　single　or　combined　mixtures　on　the　modified　concrete＇s　performance.　UMK　has　a　significant　impact　on　the　short-term　mechanical　performance,　while　NT　showed　long-term　contribution　to　the　performance　under　the　corrosive　environment.