The　response　characteristics　of　large-diameter　monopiles　under　lateral　loads　greatly　differ　from　those　of　traditional　small-diameter　piles.　By　means　of　transparent　soil　and　centrifuge　tests,　as　well　as　numerical　analyses,　the　soil　flow　mechanism　and　lateral　behavior　of　large-diameter　monopiles　in　soft　clay　were　investigated.　The　study　result　reveals　that　a　rotational　failure　mechanism,　different　from　wedge-full-flow　mechanisms,　exists　in　the　large-diameter　monopile–soil　system.　For　stubby　large-diameter　monopiles,　using　the　classic　wedge-full-flow　mechanisms　can　lead　to　an　overestimation　of　lateral　capability　due　to　its　absence　in　considering　the　rotational　mechanism.　In　addition,　the　normalized　p–y　curves　in　the　rotational　soil　flow　zone　are　also　observed　to　have　larger　initial　stiffness　and　attain　yield　plateau　earlier　than　those　in　wedge-full-flow　zone.　Differences　in　Np　values　among　different　centrifuge　tests　are　partly　attributed　to　the　different　means　of　obtaining　the　clay’s　undrained　shear　strength.　When　the　undrained　strength　from　consolidated　undrained　triaxial　tests　is　taken,　consistent　values　of　Np　originating　from　different　test　results　are　observed.　Finally,　based　on　the　results　from　centrifuge　tests　and　well-calibrated　numerical　analyses,　an　empirical　formulation　of　Np　for　large-diameter　monopiles　in　soft　clay　is　proposed.　©　The　Author(s),　under　exclusive　licence　to　Springer-Verlag　GmbH　Germany,　part　of　Springer　Nature　2024.