How can I create a window with transparent background with swift on osx?

NSWindow has a property 'opaque' which it is true by default.

The value of this property is true when the window is opaque; otherwise, false.

Just change it to false:

override func viewWillAppear() {
    super.viewWillAppear()
    view.window?.opaque = false
    view.window?.backgroundColor = NSColor(red: 1, green: 0.5, blue: 0.5, alpha: 0.5)
}

Swift 4 update: opaque has been renamed isOpaque

override func viewWillAppear() {
    super.viewWillAppear()
    view.window?.isOpaque = false
    view.window?.backgroundColor = NSColor(red: 1, green: 0.5, blue: 0.5, alpha: 0.5) 
}

Make the window non-opaque, and give it a clear background:

func applicationDidFinishLaunching(aNotification: NSNotification) {
    window.opaque = false
    window.backgroundColor = NSColor.clearColor()
}

A little update for Swift 3

A window subclass example with comment:

class customWindow: NSWindow {
    
    override init(contentRect: NSRect, styleMask style: NSWindowStyleMask, backing bufferingType: NSBackingStoreType, defer flag: Bool) {
        super.init(contentRect: contentRect, styleMask: style, backing: bufferingType, defer: flag)
        
        // Set the opaque value off,remove shadows and fill the window with clear (transparent)
        self.isOpaque = false
        self.hasShadow = false
        self.backgroundColor = NSColor.clear
        
        // Change the title bar appereance
        self.title = "My Custom Title"
        //self.titleVisibility = .hidden
        self.titlebarAppearsTransparent = true


        
        
    }
    

• More on Title Bar appereance here Title Bar and Toolbar Showcase