In this study, we introduce a cost-effective antenna designed for ultrawideband (UWB) spectrum. The antenna employs a single-layer coplanar waveguide (CPW) feed on a 1.6 mm FR4 board. Notably, the antenna demonstrates a bandwidth from 3.7 to 15.7 GHz (12 GHz) for a single element. Expanding its utility, a four-element Multiple Input Multiple Output (MIMO) assembly is arranged orthogonally, aiming to achieve diversity characteristics. The MIMO elements are connected through a common ground plane. In the MIMO configuration, the bandwidth response is increased to 2.9–16.2 GHz. To mitigate mutual coupling between the antenna elements, a simple geometric parasitic element is inserted between them, resulting in a significant improvement of 15 dB in isolation. In addition, a maximum gain of 5.5 dBi is noted at 14 GHz with total efficiency exceeding 80% throughout the resonance bandwidth. The individual antenna element boasts a compact footprint of 20 × 26 mm2, while the MIMO configuration occupies an area of 46 × 48 mm2. Furthermore, we conduct a thorough analysis of various MIMO performance metrics, including the envelope correlation coefficient (ECC), mean effective gain (MEG), and diversity gain (DG), all of which fall within acceptable thresholds. These analyses validate the potential of our proposed UWB–MIMO antenna for diverse UWB applications.