Ultra-high　performance　seawater　sea　sand　concrete　(UHPSSC)　is　an　advanced　cementitious　composite　material　with　significant　potential　for　coastal　and　marine　infrastructure　applications.　Marine　tidal　zones　represent　one　of　the　most　aggressive　environments　for　concrete　structures　due　to　coupled　chemical-physical　degradation　mechanisms.　This　study　aims　to　elucidate　the　long-term　strength　evolution　mechanisms　of　UHPSSC　under　marine　tidal　zone　exposure.　Specifically,　the　mass　loss,　mechanical　properties,　and　chemically　bound　water　content　of　UHPSSC　and　ultra-high　performance　concrete　(UHPC)　are　evaluated　after　exposure　to　seawater/freshwater　freeze-thaw　cycles　and　seawater/freshwater　dry-wet　cycles.　Microstructural　characterization　was　employed　to　elucidate　the　underlying　strength　evolution　mechanisms.　The　results　demonstrate　that　the　coupling　of　damage　and　rehydration　governs　the　strength　evolution　of　UHPSSC　and　UHPC　under　tidal　zone　conditions.　Following　1000　freeze-thaw　cycles　and　360　dry-wet　cycles,　the　mechanical　properties　and　the　chemically　bound　water　content　of　UHPSSC　and　UHPC　increased　slightly,　but　the　flexural　strength　under　seawater　dry-wet　cycling　decreased.　In　particular,　after　360　freshwater-dry-wet　cycles,　the　compressive　strength,　flexural　strength,　and　chemically　bound　water　content　of　UHPSSC　increased　by　18.7　%,　14.2　%,　and　21　%,　respectively.　For　UHPC,　the　corresponding　increases　were　16.2　%,　30.6　%,　and　20　%,　respectively.　The　effect　of　tidal　zone　erosion　on　the　mass　was　negligible,　with　a　maximum　of　no　more　than　0.9　%.　Remarkably,　even　after　1000　freeze-thaw　cycles　and　360　dry-wet　cycles,　the　steel　fibers　within　the　UHPSSC　matrix　were　uncorroded.　Both　UHPSSC　and　UHPC　exhibited　exceptional　erosion　resistance　in　marine　tidal　environments.