Set the line break mode to word-wrapping and the number of lines to 0:

// Swift
textLabel.lineBreakMode = .byWordWrapping
textLabel.numberOfLines = 0

// Objective-C
textLabel.lineBreakMode = NSLineBreakByWordWrapping;
textLabel.numberOfLines = 0;

// C# (Xamarin.iOS)
textLabel.LineBreakMode = UILineBreakMode.WordWrap;
textLabel.Lines = 0;  

Restored old answer (for reference and devs willing to support iOS below 6.0):

textLabel.lineBreakMode = UILineBreakModeWordWrap;
textLabel.numberOfLines = 0;

On the side: both enum values yield to 0 anyway.