_TheForm.cshtml

@using WebApplication1.Models
@model AjaxTestIndexModel

<div class="row">
    <div class="form-group">
        <label class="col-sm-2 control-label">
            @Html.DisplayNameFor(m => m.Name)
        </label>
        <div class="col-sm-6">
            @Html.TextBoxFor(m => m.Name)
        </div>
    </div>
    <div class="form-group">
        <label class="col-sm-2 control-label">
            @Html.DisplayNameFor(m => m.Subject)
        </label>
        <div class="col-sm-6">
            @Html.TextBoxFor(m => m.Subject)
        </div>
    </div>
</div>

<div class="alert alert-danger">
    @Html.ValidationSummary()
</div>

<div class="buttons">
    <button type="submit" class="btn btn-primary">Submit</button>
</div>

AjaxTestController

public class AjaxTestController : Controller
{
    [HttpGet]
    public ActionResult Index()
    {
        return View();
    }
    
    [HttpPost]
    public ActionResult PostTest(AjaxTestIndexModel model)
    {
        if (model == null)
        {
            throw new ArgumentNullException("model");
        }

        if (!ModelState.IsValid)
        {
            return PartialView("_TheForm", model);
        }


        return new JsonResult
        {
            Data = new { Success = true }
        };
    }
}

AjaxTestFormModel

public class AjaxTestFormModel
{
    [Required]
    [Display(Name = "Name")]
    public string Name { get; set; }

    [Required]
    [Display(Name = "Subject")]
    public string Subject { get; set; }
}

Index.cshtml

@{
    ViewBag.Title = "Ajax Test";

    var options = new AjaxOptions()
    {
        Url = Url.Action("PostTest", "AjaxTest"),
        HttpMethod = "POST",
        UpdateTargetId = "update-target",
        OnSuccess = "alert('success');",
        OnFailure = "alert('fail');"
    };
}

<style type="text/css">
    .form-group {
        clear: both;
        overflow: hidden;
    }
</style>

<h2>@ViewBag.Title</h2>
<div class="update-target" id="update-target">
    @using(Ajax.BeginForm(options))
    {
        @Html.Partial("_TheForm")    
    }
</div>

Partial _Layout.cshtml

...

    @Scripts.Render("~/bundles/jquery")
    @Scripts.Render("~/bundles/bootstrap")
    @Scripts.Render("~/bundles/jqueryval") // Add this line!

...

Partial BundleConfig

...

bundles.Add(new ScriptBundle("~/bundles/jqueryval").Include(
                        "~/Scripts/jquery.unobtrusive*", // Add this line!
                        "~/Scripts/jquery.validate*"));
                        
...