An orthosis to be placed on the hand of a user, including a supporting member having the general shape of a horse saddle and having an inner flange and an outer flange, to be placed at the thumb-index commissure with the inner flange against the palm and the outer wing against the back surface of the hand, in order to enable a spacing to be maintained between the thumb and the index finger. The orthosis includes a strap to be placed around the hand and the supporting member so as to retain said supporting member such that the metacarpal bone of the thumb is trapped inside the orthosis between the strap and supporting member.
