Automated and robust separation of 14 μl of plasma from 40 μl of whole blood at a purity of 99.81% ± 0.11% within 43 s is demonstrated for the hematocrit range of 20%-60% in a centrifugal microfluidic polymer disk. At high rotational frequency, red blood cells (RBCs) within whole blood are concentrated in a radial outer RBC collection chamber. Simultaneously, plasma is concentrated in a radial inner pneumatic chamber, where a defined air volume is enclosed and compressed. Subsequent reduction of the rotational frequency to not lower than 25 Hz enables rapid transfer of supernatant plasma into a plasma collection chamber, with highly suppressed resuspension of red blood cells. Disk design and the rotational protocol are optimized to make the process fast, robust, and insusceptible for undesired cell resuspension. Numerical network simulation with lumped model elements is used to predict and optimize the fluidic characteristics. Lysis of the remaining red blood cells in the purified plasma, followed by measurement of the hemoglobin concentration, was used to determine plasma purity. Due to the pneumatic actuation, no surface treatment of the fluidic cartridge or any additional external means are required, offering the possibility for low-cost mass fabrication technologies, such as injection molding or thermoforming.