A variable volume infusion port is provided. The infusion port may include a port body having an internal fluid reservoir, a septum, a stem to fluidly couple to a catheter lumen, and a displaceable member disposed in the internal fluid reservoir. In a first position, the displaceable member is disposed proximate the septum, providing a relatively small fluid volume within the infusion port. Insertion of an injection device through the septum causes the displaceable member to move to a second location distal from the septum, providing a relatively large fluid volume within the infusion port for the duration the injection device remains in the infusion port. The displaceable member may include a rigid member operably coupled to a biasing element or a flexible member coupled to a biasing element.