An orthosis for fastening to a human hand for immobilizing the carpometacarpal joint and metacarpophalangeal joint of the thumb of the hand, comprising a stiff body (2, 52, 102) with a metacarpal section (5, 55, 105) which is formed by a section of a curved and stiff element and which - has a first end (5a, 55a, 105a) and a second end (5b, 55b, 105b), which are spaced apart from each other, and - is designed in such a manner that said metacarpal section can be brought into engagement with a human hand by being placed against the palm of the hand below the metacarpophalangeal joints of an index finger, middle finger, ring finger and little finger such that it extends in a direction between the two edges of the hand over at least part of the palm and at least partially rests against the latter, and a stiff thumb section (6, 56, 106) which - extends from the metacarpal section (5, 55, 105) and has a first end (6a, 56a, 106a), which is fastened to the metacarpal section (5, 55, 105) or is formed by part of the metacarpal section (5, 55, 105), and a second end (6b, 56b, 106b), - in the region of its second end (6b, 56b, 106b), has a ring-shaped or ring-segment-shaped thumb holding section (7, 57, 107) and - is designed in such a manner that when the metacarpal section (5, 55, 105) is brought into engagement with the hand, the thumb section (6, 56, 106) can be placed at the thumb of the hand such that - the thumb is arranged in the ring opening (10, 60, 110) defined by the thumb holding section (7, 57, 107), and the thumb holding section (7, 57, 107) extends over at least part of the circumference of the thumb and at least partially surrounds the latter and - supports the thumb against a movement in the direction of the palm in a position in which the thumb and the index finger of the hand are put opposite each other and the index finger can be moved toward the thumb into contact therewith and can be moved away from the thumb, wherein the metacarpal section is designed in such a manner that, when the orthosis is placed onto a human hand and the metacarpal section has been brought into engagement with the hand and the thumb section has been placed against the thumb of the hand such that the thumb is arranged in the ring opening defined by the thumb holding section and the thumb holding section extends over at least part of the circumference of the thumb and at least partially surrounds the latter, said metacarpal section rests against the hand exclusively in the region of the palm, and wherein the orthosis furthermore has a closure device (170a, 170b) for fastening the orthosis when placed on the hand, wherein the closure device (170a, 170b) has at least one first fastening section which is arranged and configured in such a manner that, when the orthosis is placed on, said fastening section can be arranged and secured on the metacarpal section and/or the thumb section in such a manner that said fastening section runs over the back of the hand and, together with the metacarpal section, forms at least part of an annularly closed section of the orthosis, which section surrounds the hand, and wherein the metacarpal section has a first surface for contact with the palm and an opposite, second surface which, when the orthosis is put on, is directed away from the hand, wherein the first surface has two concave sections (151a, 151b) and a convex section arranged between the latter, between the first end and the second end of the metacarpal section. - has a first end (5a, 55a, 105a) and a second end (5b, 55b, 105b), which are spaced apart from each other, and - is designed in such a manner that said metacarpal section can be brought into engagement with a human hand by being placed against the palm of the hand below the metacarpophalangeal joints of an index finger, middle finger, ring finger and little finger such that it extends in a direction between the two edges of the hand over at least part of the palm and at least partially rests against the latter, and - extends from the metacarpal section (5, 55, 105) and has a first end (6a, 56a, 106a), which is fastened to the metacarpal section (5, 55, 105) or is formed by part of the metacarpal section (5, 55, 105), and a second end (6b, 56b, 106b), - in the region of its second end (6b, 56b, 106b), has a ring-shaped or ring-segment-shaped thumb holding section (7, 57, 107) and - is designed in such a manner that when the metacarpal section (5, 55, 105) is brought into engagement with the hand, the thumb section (6, 56, 106) can be placed at the thumb of the hand such that - the thumb is arranged in the ring opening (10, 60, 110) defined by the thumb holding section (7, 57, 107), and the thumb holding section (7, 57, 107) extends over at least part of the circumference of the thumb and at least partially surrounds the latter and - supports the thumb against a movement in the direction of the palm in a position in which the thumb and the index finger of the hand are put opposite each other and the index finger can be moved toward the thumb into contact therewith and can be moved away from the thumb,