Input validation is crucial to ensure the integrity and security of data received from clients in a Web API. In ASP.NET Core, you can perform input validation using data annotations, model validation, and custom validation rules. Data annotations such as Required, Range, RegularExpression, and others can be applied to model properties to define validation rules. Model binding automatically performs validation based on these annotations. You can also manually validate input data using the ModelState.IsValid property and return appropriate error responses if validation fails. Implementing input validation protects the API from receiving invalid or malicious data.