A polyaxial bone anchoring device comprising a bone anchoring element comprising a shank for anchoring to a bone and a head , a coupling device for coupling a bone anchor to a rod, the coupling device including a receiving part comprising a first end , a second end , a longitudinal axis (c) extending through the first end and the second end, a channel at the first end for receiving a rod, and an accommodation space for pivotably holding a head of the bone anchor ; a pressure member configured to be positioned in the receiving part and to exert pressure onto the inserted head to lock the head in the receiving part, the pressure member comprising a cap-like portion having a pressure exerting surface and a deformable portion comprising a free end , wherein the deformable portion is configured to assume at least a first configuration and a second configuration, and wherein the pressure member is movable along the longitudinal axis from an insertion position to a locking position and wherein the cap-like portion is expandable in the accommodation space in the insertion position; and a locking member configured to be inserted into the channel to move in the channel along the longitudinal axis and to exert a force onto the deformable portion of the pressure member; wherein the deformable portion is elastically deformable, characterised in that when the head is in the receiving part, and when the deformable portion of the pressure member is in the first configuration and the locking member is in contact with the deformable portion at a first axial position relative to the receiving part, the free end of the deformable portion is supported by the receiving part and the locking member is moveable towards the head to exert a force on the deformable portion that deforms the deformable portion from the first configuration to the second configuration, and wherein the deformable portion transforms from the first, angled configuration into the second, less angled or substantially straight configuration, wherein the pressure exerted by the pressure member onto the head increases. a receiving part comprising a first end , a second end , a longitudinal axis (c) extending through the first end and the second end, a channel at the first end for receiving a rod, and an accommodation space for pivotably holding a head of the bone anchor ; a pressure member configured to be positioned in the receiving part and to exert pressure onto the inserted head to lock the head in the receiving part, the pressure member comprising a cap-like portion having a pressure exerting surface and a deformable portion comprising a free end , wherein the deformable portion is configured to assume at least a first configuration and a second configuration, and wherein the pressure member is movable along the longitudinal axis from an insertion position to a locking position and wherein the cap-like portion is expandable in the accommodation space in the insertion position; and a locking member configured to be inserted into the channel to move in the channel along the longitudinal axis and to exert a force onto the deformable portion of the pressure member; wherein the deformable portion is elastically deformable, <EOS>