An implantable liner device for preventing formation of blood clots in a left atrium of a patient's heart. The liner device includes a liner body configured, in some embodiments, to cover a portion of an interior surface of the left atrium, the liner body forming a first opening sized for placement about pulmonary vein ostiums of the left atrium and a second opening for fluid alignment with a mitral valve of the left atrium. The liner body can self-expandable from a collapsed state appropriate for percutaneous delivery to the atrium to an expanded state, with the expanded state having a shape appropriate for covering the portion of the interior surface of the left atrium. The liner body can have a cap-like shape, and can be formed of mesh, fabric, braided material, and biological material such as tissue.