An implantable device intended to divert excessive cerebrospinal fluid (CSF) from the subarachnoid space of the lumbar spine to the peritoneal cavity where it can be absorbed into the body, as a treatment for raised CSF pressure (e.g., caused by hydrocephalus). It consists of fully-implantable interconnected components: a spinal catheter implanted into subarachnoid space of the lumbar spine; a mechanical shunt valve that may incorporate a puncturable port/reservoir for sampling/infusion; a peritoneal catheter implanted into the peritoneal cavity; some types incorporate a subcutaneous anchor for component connection/positioning. Disposable devices associated with implantation may be included.