A novel microstrip parallel coupler with tunable coupling coefficient is proposed. Two varactors are placed on microstrip parallel-coupled lines to tune the coupling coefficient. With proper combinations of the capacitances of two varactors, the coupling coefficient can be tuned from 10 dB to 6 dB with good return loss and directivity. By using the Lab view program and data acquisition component, a software configurable coupler is implemented. This software configurable coupler will be useful in microwave instrumentation.