The programming language primarily used for developing iOS applications is Swift. Introduced by Apple, Swift is designed for safety and performance, making it efficient for mobile app development. Its syntax is user-friendly and modern, allowing developers to write code more quickly and with fewer errors compared to its predecessor, Objective-C. Swift offers various features like type inference, functional programming capabilities, and robust libraries, which makes it a preferred choice for creating interactive and high-performance iOS apps on the Apple ecosystem.