This paper presents a detailed investigation utilizing a defected ground structure (DGS) to a conventional edge-coupled microstrip coupler with tight coupling level over a relatively wide frequency band. A reasonable spacing between microstrip coupled lines and stronger coupling are achieved using this technique. A 20 dB initial coupler (over a fractional bandwidth of 35% with 1dB ripple) is converted into 8 dB coupler with almost 3 times wider band by etching off a single unit cell of rectangular and meandered slot loop in the ground plane under the central part of the coupling region. Optimum DGS dimensions are related to coupler dimensions in easy to use design curves. An efficient technique for compensation of the significant unavoidable mismatch resulting from the presence of the DGS is applied and tested. Some coupler samples with different DGS are fabricated, measured, and compared with a conventional coupler counterpart to verify the simulation results and illustrate the improvements, very good agreements are observed.