Protective cap for capillary writing instruments comprised of a relatively flexible outer casing and a relatively rigid inner casing. One end of the outer casing is adapted to form a substantially fluid tight seal on the body of a writing instrument. A flexible sealing means, e.g., spherical, is positioned between the inner sheath and the outer sheath so as to form a substantially fluid tight seal on the tip of a writing instrument inserted into the inner casing and placed in contact with the sealing means. In another embodiment the sealing means located between the outer casing and inner casing is eliminated and the outer casing forms a substantially fluid tight seal by directly contacting the writing instrument tip.