The significance of responsive design in modern web development lies in its ability to provide an optimal user experience across a wide range of devices, including smartphones, tablets, and desktops. By using flexible layouts, images, and CSS media queries, responsive design ensures that websites adapt seamlessly to different screen sizes and orientations. This adaptability enhances usability and accessibility, allowing users to navigate content easily, regardless of their device. Additionally, responsive design plays a crucial role in search engine optimization (SEO), as search engines favor mobile-friendly websites, potentially increasing visibility and traffic.