An automatic sealing for a window or door leaf, wherein the leaf has at least one substantially horizontal end edge with a longitudinal groove, wherein the sealing comprises: - a fixed supporting rail insertable in the longitudinal groove of a window or door leaf and having a substantially longitudinal U-shaped portion defining an internal seat; - a movable insulating profile slidably housed into said seat and having a sealing member made of a first elastomeric material and designed for selectively pressing against a floor or threshold; - actuating means movable along a vertical direction to move outwardly said sealing member between an inoperative position completely outwardly inside said seat and an operative position at least partially projecting outwardly from said seat; wherein said insulating profile further comprises an interface member made of a substantially rigid material with an external side at least partially in constant contact relationship with said sealing member and an internal side susceptible to interact with said actuating means to transmit the translation (t) of said actuating means to said sealing member; wherein said sealing member is substantially U-shaped with a pair of first lateral arms (11, 11') joined by a central portion susceptible to be pressed against a floor or threshold; wherein said interface member has a pair of second substantially vertical lateral arms (13, 13') associated to corresponding first lateral arms (11, 11') of said sealing member and a substantially horizontal central section designed to interact with said actuating means and with said central portion of said sealing member; wherein said actuating means comprise a pushing rail housed into said seat and having a U-shaped upper portion susceptible to interact with an external control mechanism associated to the leaf; characterized in that said second lateral arms (13, 13') are joined to said central section 14) by means of respective elastically yieldable joints (15, 15'), said pushing rail having a substantially C-shaped lower portion susceptible to interact with said central section of said interface member, said second lateral arms (13, 13') having, at said joints (15, 15'), diverging portions (21, 21') defining respective inclined planes designed to interact with said C-shaped lower portion of said pushing rail so that upon translation (t) of this latter from said rest position to said operating position, said second lateral arms (13, 13') are caused to rotate about said joints (15, 15') simultaneously causing said first side arms (11, 11') to selectively interact with the inner sidewalls of said seat and transmit without delay the translation of said actuating means to said sealing member. - a fixed supporting rail insertable in the longitudinal groove of a window or door leaf and having a substantially longitudinal U-shaped portion defining an internal seat; - a movable insulating profile slidably housed into said seat and having a sealing member made of a first elastomeric material and designed for selectively pressing against a floor or threshold; - actuating means movable along a vertical direction to move outwardly said sealing member between an inoperative position completely outwardly inside said seat and an operative position at least partially projecting outwardly from said seat;