A skin-mountable drug delivery device (1) is provided comprising a transcutaneous device unit (2) and a reservoir unit (5). The transcutaneous device unit comprises a transcutaneous needle for transporting a fluid through a skin portion of a subject, and a mounting surface adapted for application to the skin of the subject. The reservoir unit comprises a reservoir adapted to contain a fluid drug, the reservoir comprising an outlet means allowing the transcutaneous needle to be arranged in fluid communication with an interior of the reservoir, and an expelling assembly for expelling a fluid drug out of the reservoir and through the skin of the subject via the transcutaneous needle. The transcutaneous device unit and the reservoir unit further comprise coupling means allowing the reservoir unit to be secured to the transcutaneous device unit in the situation of use. By this arrangement a two-unit system is provided which can be used in a convenient and cost-effective manner.
