A sterile device designed to be used with a transseptal introducer sheath to penetrate the interatrial septum to gain left-heart access during a transseptal catheterization procedure. It is typically a long, thin tube made of plastic that is curved and round-nosed at the distal end to accommodate positioning against the atrial septum, and to accommodate in its lumen a Brockenbrough type curved puncture needle. It will typically have a marker (e.g., a hub) to identify the proximal portion of the dilator and to aid in placement of the sheath. This is a single-use device.