WKWebView in Interface Builder

You are correct - it doesn't seem to work. If you look in the headers, you'll see:

- (instancetype)initWithCoder:(NSCoder *)coder NS_UNAVAILABLE;

which implies that you can't instantiate one from a nib.

You'll have to do it by hand in viewDidLoad or loadView.

As pointed out by some, as of Xcode 6.4, WKWebView is still not available on Interface Builder. However, it is very easy to add them via code.

I'm just using this in my ViewController. Skipping Interface builder

import UIKit
import WebKit

class ViewController: UIViewController {

    private var webView: WKWebView?

    override func loadView() {
        webView = WKWebView()

        //If you want to implement the delegate
        //webView?.navigationDelegate = self

        view = webView
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        if let url = URL(string: "https://google.com") {
            let req = URLRequest(url: url)
            webView?.load(req)
        }
    }
}

Info.plist

add in your Info.plist transport security setting

 <key>NSAppTransportSecurity</key>
 <dict>
    <key>NSAllowsArbitraryLoads</key>
    <true/>
 </dict>

Xcode 9.1+

Using interface builder

You can find WKWebView element in the Object library.

Add view programmatically with Swift 5

let webView = WKWebView(frame: .zero, configuration: WKWebViewConfiguration())
view.addSubview(webView)
webView.translatesAutoresizingMaskIntoConstraints = false
webView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor).isActive = true
webView.rightAnchor.constraint(equalTo: view.safeAreaLayoutGuide.rightAnchor).isActive = true
webView.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor).isActive = true
webView.leftAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leftAnchor).isActive = true

Add view programmatically with Swift 5 (full sample)

import UIKit
import WebKit

class ViewController: UIViewController {

    private weak var webView: WKWebView!

    override func viewDidLoad() {
        super.viewDidLoad()
        initWebView()
        webView.loadPage(address: "http://apple.com")
    }

    private func initWebView() {
        let webView = WKWebView(frame: .zero, configuration: WKWebViewConfiguration())
        view.addSubview(webView)
        self.webView = webView
        webView.translatesAutoresizingMaskIntoConstraints = false
        webView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor).isActive = true
        webView.rightAnchor.constraint(equalTo: view.safeAreaLayoutGuide.rightAnchor).isActive = true
        webView.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor).isActive = true
        webView.leftAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leftAnchor).isActive = true
    }
}

extension WKWebView {
    func loadPage(address url: URL) { load(URLRequest(url: url)) }
    func loadPage(address urlString: String) {
        guard let url = URL(string: urlString) else { return }
        loadPage(address: url)
    }
}

With Xcode 8 this is now possible, but the means of achieving it is a little hacky to say the least. But hey, a working solution is a working solution, right? Let me explain.

WKWebView's initWithCoder: is no longer annotated as "NS_UNAVAILABLE". It now looks as shown below.

- (nullable instancetype)initWithCoder:(NSCoder *)coder NS_DESIGNATED_INITIALIZER;

Start by subclassing WKWebView and override initWithCoder. Instead of calling super initWithCoder, you'll need to use a different init method, such as initWithFrame:configuration:. Quick example below.

- (instancetype)initWithCoder:(NSCoder *)coder
{
    // An initial frame for initialization must be set, but it will be overridden 
    // below by the autolayout constraints set in interface builder. 
    CGRect frame = [[UIScreen mainScreen] bounds];
    WKWebViewConfiguration *myConfiguration = [WKWebViewConfiguration new];

    // Set any configuration parameters here, e.g.
    // myConfiguration.dataDetectorTypes = WKDataDetectorTypeAll; 

    self = [super initWithFrame:frame configuration:myConfiguration];

    // Apply constraints from interface builder.
    self.translatesAutoresizingMaskIntoConstraints = NO;

    return self;
}

Over in your Storyboard, use a UIView and give it a custom class of your new subclass. The rest is business as usual (setting auto-layout constraints, linking the view to an outlet in a controller, etc).

Finally, WKWebView scales content differently to UIWebView. Many people are likely going to want to follow the simple advice in Suppress WKWebView from scaling content to render at same magnification as UIWebView does to make WKWebView more closely follow the UIWebView behaviour in this regard.