A new polymer microchip is presented in this paper for separation and particle sorting in flowing suspensions. It includes a microchannel where a fluid-sample containing particles flows in parallel with another liquid. The working principle is based on an ultrasonic actuation on the cross-section of the chip, which behaves as a multilayer system. That part of the wave established within the channel includes a node of pressure strategically located within the collector fluid path, where the target particles collected to be extracted from their host sample. The use of a polymer as the constitutive material of the chip, new in this type of devices, allows a channel width somewhat larger than a quarter of a wavelength, which does not fit any of the conventional models but it represents an intermediate situation. The device has been fabricated by standard SU-8 photolithography using PMMA as substrate. In the experiments, polystyrene particles of 20 μm in diameter have been extracted from aqueous suspensions containing 6-μm sized particles by a strategic application of ultrasonic waves. A high efficiency of particle separation, over 95%, at different concentrations and flow rates, prove the feasibility of the device to carry out sorting processes on flowing suspensions.