This paper presents a compact ultrawideband slotted patch antenna for early stage tumor detection applications using microwave breast imaging systems. In order to reduce any reflections between the antenna and breast, the antenna is designed and optimized inside a coupling medium with electrical properties similar to that of normal breast tissue as a coupling medium. The wideband performance of the antenna is obtained by adding a rectangular slit and L-shaped sleeves on the ground plane. While the added rectangular slit mainly improves the matching, the L-shaped sleeves improve both the matching and the bandwidth. The antenna operates from 0.9 to 9.6 GHz with a fractional bandwidth of ~165$\%$, providing a good penetration and resolution. In addition, the proposed antenna has a very compact size of 16 × 16 mm2. Furthermore, an antenna array consisting of 31 antennas is formed around a realistic breast phantom in the simulation environment to demonstrate the imaging of a spherical tumor with a diameter of 5 mm inside the breast. The results confirm that the proposed antenna can be used for early stage breast tumor detection purposes.