UISegmentedControl.appearance().setTitleTextAttributes(NSDictionary(objects: [UIFont.systemFont(ofSize: 16.0)],
                                                                        forKeys: [kCTFontAttributeName as! NSCopying]) as? [AnyHashable : Any],
                                                           for: UIControlState.normal)

斯威夫特风格:

UISegmentedControl.appearance().setTitleTextAttributes(NSDictionary(objects: [UIFont.systemFontOfSize(14.0)], forKeys: [NSFontAttributeName]), forState: UIControlState.Normal)

XCode 8.1, Swift 3

import UIKit
class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        UISegmentedControl.appearance().setTitleTextAttributes(NSDictionary(objects: [UIFont.systemFont(ofSize: 24.0)], 
        forKeys: [NSFontAttributeName as NSCopying]) as? [AnyHashable : Any], 
        for: UIControlState.normal)
    }
}

只需要改变数值(ofSize: 24.0)

C# / Xamarin：

segment.SetTitleTextAttributes(new UITextAttributes { 
    Font = UIFont.SystemFontOfSize(font_size) }, UIControlState.Normal);

在swift 5中，

let font = UIFont.systemFont(ofSize: 16)
UISegmentedControl.appearance().setTitleTextAttributes([NSAttributedString.Key.font: font], for: .normal)

以下是Swift版本的公认答案:

斯威夫特3:

let font = UIFont.systemFont(ofSize: 16)
segmentedControl.setTitleTextAttributes([NSFontAttributeName: font],
                                        for: .normal)

斯威夫特2.2:

let font = UIFont.systemFontOfSize(16)
segmentedControl.setTitleTextAttributes([NSFontAttributeName: font], 
    forState: UIControlState.Normal)