How do I clear MVC client side validation errors when a cancel button is clicked when a user has invalidated a form?

This answer is for MVC3. See comments below for help updating it to MVC 4 and 5

If you just want to clear the validation-messages so that they are not shown to the user you can do it with javascript like so:

function resetValidation() {
        //Removes validation from input-fields
        $('.input-validation-error').addClass('input-validation-valid');
        $('.input-validation-error').removeClass('input-validation-error');
        //Removes validation message after input-fields
        $('.field-validation-error').addClass('field-validation-valid');
        $('.field-validation-error').removeClass('field-validation-error');
        //Removes validation summary 
        $('.validation-summary-errors').addClass('validation-summary-valid');
        $('.validation-summary-errors').removeClass('validation-summary-errors');

    }

If you need the reset to only work in your popup you can do it like this:

function resetValidation() {
        //Removes validation from input-fields
        $('#POPUPID .input-validation-error').addClass('input-validation-valid');
        $('#POPUPID .input-validation-error').removeClass('input-validation-error');
        //Removes validation message after input-fields
        $('#POPUPID .field-validation-error').addClass('field-validation-valid');
        $('#POPUPID .field-validation-error').removeClass('field-validation-error');
        //Removes validation summary 
        $('#POPUPID .validation-summary-errors').addClass('validation-summary-valid');
        $('#POPUPID .validation-summary-errors').removeClass('validation-summary-errors');

    }

I hope this is the effect you seek.