Slender　and　flexible　offshore　wind　turbines　(OWTs)　are　vulnerable　to　external　dynamic　excitations,　and　passive　tuned　mass　dampers　(TMDs)　have　been　widely　used　to　control　excessive　vibrations　of　OWTs　under　harsh　marine　environments　(e.g.,　strong　winds　and　irregular　sea　waves).　However,　TMDs　are　only　effective　in　the　vicinity　of　the　controlled　frequency,　i.e.,　in　a　narrow　frequency　band.　Compared　to　passive　TMDs,　active　control　methods　are　normally　considered　to　possess　better　control　performances　but　at　the　cost　of　a　large　amount　of　external　energy　input.　To　this　end,　the　present　study　proposes　a　novel　energy-adaptive　self-powered　active　mass　damper　(SPAMD)　to　mitigate　the　responses　of　OWT　towers.　The　proposed　control　device　can　harvest　energies　from　OWTs　and　then　use　them　as　the　power　to　drive　an　active　mass　damper　for　structural　vibration　control.　Specifically,　a　representative　OWT　is　selected　as　a　prototype　structure　and　its　tower　is　modeled　as　a　multi-degree-of-freedom　system　by　simplifying　the　rotor-nacelle　assembly　as　a　lumped　mass　and　moment　of　inertia.　The　dynamic　characteristics　(mainly　natural　frequency　and　mode　shape)　of　the　tower　obtained　by　the　developed　model　are　validated　against　a　finite　element　model.　Subsequently,　the　system　configuration　and　working　mechanism　of　SPAMD　are　introduced　and　SPAMD　is　incorporated　into　the　developed　model　to　simultaneously　harvest　energy　and　mitigate　the　fore-aft　responses　of　the　tower　under　wind　and　sea　wave　loads.　The　control　effectiveness　of　SPAMD　is　further　compared　to　the　traditional　TMD.　Results　show　that　SPAMD　has　a　superior　effect　over　TMD　in　controlling　OWT　responses.