2021年的正确答案。语法改变了。

12岁的答案(甚至是对它的编辑)是坏的。

它只是:

let _font = UIFont.systemFont(ofSize: 10)
UISegmentedControl.appearance()
 .setTitleTextAttributes([NSAttributedString.Key.font: _font], for: .normal)

你很可能会想要正确地改变高度，而你在它:

import UIKit

class SmallerSegmentedControl: UISegmentedControl {

    override init(frame: CGRect) {
        super.init(frame: frame)
        common()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        common()
    }

    func common() {
        let _font = UIFont.systemFont(ofSize: 10)
        UISegmentedControl.appearance()
         .setTitleTextAttributes([NSAttributedString.Key.font: _font], for: .normal)
    }

    override var intrinsicContentSize:CGSize {
        var s = super.intrinsicContentSize
        s.height = 24
        return s
    }

}

C# / Xamarin：

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

在swift 5中，

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

你可以通过递归检查从UISegmentedControl开始的每个视图来获得UILabel的实际字体。我不知道这是不是最好的方法，但它确实有效。

@interface tmpSegmentedControlTextViewController : UIViewController {
}

@property (nonatomic, assign) IBOutlet UISegmentedControl * theControl;

@end

@implementation tmpSegmentedControlTextViewController

@synthesize theControl; // UISegmentedControl

- (void)viewDidLoad {
  [self printControl:[self theControl]];
  [super viewDidLoad];
}

- (void) printControl:(UIView *) view {
  NSArray * views = [view subviews];
  NSInteger idx,idxMax;
  for (idx = 0, idxMax = views.count; idx < idxMax; idx++) {
    UIView * thisView = [views objectAtIndex:idx];
    UILabel * tmpLabel = (UILabel *) thisView;
    if ([tmpLabel respondsToSelector:@selector(text)]) {
      NSLog(@"TEXT for view %d: %@",idx,tmpLabel.text);
      [tmpLabel setTextColor:[UIColor blackColor]];
    }

    if (thisView.subviews.count) {
      NSLog(@"View has subviews");
      [self printControl:thisView];
    }
  }
}

@end

在上面的代码中，我只是设置了UILabel的文本颜色，但你也可以获取或设置字体属性。

UISegmentedControl的扩展，用于设置字体大小。

extension UISegmentedControl {
    @available(iOS 8.2, *)
    func setFontSize(fontSize: CGFloat) {
            let normalTextAttributes: [NSObject : AnyObject]!
            if #available(iOS 9.0, *) {
                normalTextAttributes = [
                    NSFontAttributeName: UIFont.monospacedDigitSystemFontOfSize(fontSize, weight: UIFontWeightRegular)
                ]
            } else {
                normalTextAttributes = [
                    NSFontAttributeName: UIFont.systemFontOfSize(fontSize, weight: UIFontWeightRegular)
                ]
            }

        self.setTitleTextAttributes(normalTextAttributes, forState: .Normal)
    }
 }