Data validation ensures that the data received from clients is valid and meets the required criteria before processing or storing it. In ASP.NET Core Web API, you can implement data validation using data annotations, model validation, and the ModelState object. Data annotations, such as Required, Range, RegularExpression, and others, can be applied to model properties to define validation rules. Model binding automatically performs basic validation based on these annotations. Additionally, you can manually validate input data using the ModelState.IsValid property and return appropriate error responses if validation fails. Data validation helps maintain data integrity and prevent invalid or malicious data from entering the API.