A device for reconstituting an active ingredient in the form of sterile powder, comprising: - a vial containing said active ingredient as sterile powder having a mouth and a pierceable closing member which closes the mouth; - a flexible bag containing a sterile liquid; - a connecting component fixed to the vial and to the flexible bag; - an axially perforated spike, constrained to the connecting component and adapted to pierce the closing member, said spike comprising a tip, facing towards the closing member, and a head opposite to the tip, wherein the axially perforated spike is inserted in a hole of the connecting component, and wherein there is provided a pierceable membrane which seals said hole, characterized in that the device further comprises - a sealing member which seals the closing member at the mouth, having an open central portion and arranged so as to leave a central portion of the closing member uncovered; - an elastomeric sealing ring arranged between the connecting component and the closing member of the vial; in that the connecting component comprises an annular seat where the elastomeric sealing ring is partially accommodated, in that the elastomeric sealing ring is in contact with the sealing member; in that there is provided a hermetically sealed sterile chamber, delimited by the closing member, by the connecting component and by the elastomeric sealing ring (10, 110), the hermetically sealed sterile chamber being an empty space delimited at the top by the piercable sealing membrane and by the connecting component, laterally by the elastomeric sealing ring, and at the bottom by a part of the upper side of the sealing member and by the uncovered central portion of the closing member; in that the connecting component is provided with a plurality of coupling teeth which fix the connecting component to the vial, the coupling teeth being in abutment against the portion of sealing member that covers a step of the vial so that the elastomeric sealing ring is subjected to compression, so as to ensure the tightness of the hermetically sealed sterile chamber; in that the axially perforated spike is adapted to slide from a rest position inside the flexible bag to a working position, piercing the piercable sealing membrane before crossing the hermetically sealed sterile chamber and piercing the closing member when a pressure is exerted on the head with at least one finger of a hand, allowing the passage of the sterile liquid from the flexible bag to the vial, and of the reconstituted active ingredient from the vial to the flexible bag, whereby the active ingredient is reconstituted in sterile conditions. - a vial containing said active ingredient as sterile powder having a mouth and a pierceable closing member which closes the mouth; - a flexible bag containing a sterile liquid; - a connecting component fixed to the vial and to the flexible bag; - an axially perforated spike, constrained to the connecting component and adapted to pierce the closing member, said spike comprising a tip, facing towards the closing member, and a head opposite to the tip, - a sealing member which seals the closing member at the mouth, having an open central portion and arranged so as to leave a central portion of the closing member uncovered; - an elastomeric sealing ring arranged between the connecting component and the closing member of the vial;