Despite the growing interest in reconfigurable intelligent surfaces (RISs) for millimeter-wave (mm-wave) bands, and the considerable theoretical work reported by the communication community, there is a limited number of published works demonstrating practical implementations and experimental results. To the authors’ knowledge, no published literature has reported experimental results for RISs covering the n257 and n258 mm-wave bands. In this work, we propose a novel wideband RIS design that covers the entire mm-wave 5G n257 and n258 bands. In simulations, the unit cell can maintain a phase difference of 180° ±20° and a reflection magnitude greater than −2.8 dB within 22.7 to 30.5 GHz (29.3% bandwidth) using 1-bit p-i-n switches. The proposed unit cell design with four circular cutouts and long vias could realize wideband performance by exciting two adjacent high-order resonances (2.5 and $3.5~f$ ). The periodic unit cells can maintain an angular stability of ±30°. Based on the proposed unit cell, a $20 \times 20$ RIS array is designed and fabricated with a size of $7.1\lambda \times 7.1\lambda$ . The measurement results demonstrate that the proposed RIS could maintain a 3 dB peak gain variation bandwidth among various array configurations within 22.5 to 29.5 GHz (26.9%) and with a beam scanning capability of 50°, making this design a good candidate for 5G mm-wave applications.