Dependency injection (DI) is a core feature of ASP.NET Core that enables loose coupling and promotes maintainable and testable code. In ASP.NET Core Web API, you can implement dependency injection by registering dependencies in the service collection during application startup. This can be done in the ConfigureServices method of the Startup class. By injecting dependencies through constructor parameters or property injection, the framework resolves and provides the required dependencies to your API controllers and other components. DI simplifies unit testing, improves code reusability, and allows for easy swapping of implementations.