Swift Custom NavBar Back Button Image and Text

You can do something like that:

let yourBackImage = UIImage(named: "back_button_image")
self.navigationController?.navigationBar.backIndicatorImage = yourBackImage
self.navigationController?.navigationBar.backIndicatorTransitionMaskImage = yourBackImage
self.navigationController?.navigationBar.backItem?.title = "Custom"

Your image will only have one color though

Note: Please remember that the back button belongs to the the source ViewController and not to the destination ViewController. Thus, the modification needs to be done in the source VC, which is reflected to all the view in the navigation controller

Code Snippet:

let backImage = UIImage(named: "icon-back")

self.navigationController?.navigationBar.backIndicatorImage = backImage

self.navigationController?.navigationBar.backIndicatorTransitionMaskImage = backImage

/*** If needed Assign Title Here ***/
self.navigationItem.backBarButtonItem = UIBarButtonItem(title: "", style: UIBarButtonItemStyle.plain, target: nil, action: nil)

swift 4

In my case, I needed to have only the image of the button, without any text. I hope this will be useful to someone.

let imgBackArrow = UIImage(named: "back_arrow_32")

navigationController?.navigationBar.backIndicatorImage = imgBackArrow
navigationController?.navigationBar.backIndicatorTransitionMaskImage = imgBackArrow

navigationItem.leftItemsSupplementBackButton = true
navigationItem.leftBarButtonItem = UIBarButtonItem(title: "", style: .plain, target: self, action: nil)

For iOS 12 you can do

func setNavigationBar() {

    self.navigationItem.setHidesBackButton(true, animated:false)

    //your custom view for back image with custom size
    let view = UIView(frame: CGRect(x: 0, y: 0, width: 40, height: 40))
    let imageView = UIImageView(frame: CGRect(x: 10, y: 10, width: 20, height: 20))

    if let imgBackArrow = UIImage(named: "icn_back_arrow") {
        imageView.image = imgBackArrow
    }
    view.addSubview(imageView)

    let backTap = UITapGestureRecognizer(target: self, action: #selector(backToMain))
    view?.addGestureRecognizer(backTap)

    let leftBarButtonItem = UIBarButtonItem(customView: view ?? UIView())
    self.navigationItem.leftBarButtonItem = leftBarButtonItem
}

@objc func backToMain() {
    self.navigationController?.popViewController(animated: true)
}