How can I add a boolean value to a NSDictionary?

Well, for integers I would use NSNumber. But YES and NO aren't objects, I guess. A.f.a.i.k. I can only add objects to an NSDictionary, right?

I couldn't find any wrapper class for booleans. Is there any?

You use NSNumber.

It has init... and number... methods that take booleans, just as it does integers and so on.

From the NSNumber class reference:

// Creates and returns an NSNumber object containing a 
// given value, treating it as a BOOL.
+ (NSNumber *)numberWithBool:(BOOL)value

and:

// Returns an NSNumber object initialized to contain a
// given value, treated as a BOOL.
- (id)initWithBool:(BOOL)value

and:

// Returns the receiver’s value as a BOOL.
- (BOOL)boolValue

The new syntax since Apple LLVM Compiler 4.0

dictionary[@"key1"] = @(boolValue);
dictionary[@"key2"] = @YES;

The syntax converts BOOL to NSNumber, which is acceptable to NSDictionary.

If you are declaring it as a literal and you are using clang v3.1 or above, you should use @NO / @YES if you are declaring it as a literal. E.g.

NSMutableDictionary* foo = [@{ @"key": @NO } mutableCopy];
foo[@"bar"] = @YES;

For more info on that:

http://clang.llvm.org/docs/ObjectiveCLiterals.html

As jcampbell1 pointed out, now you can use literal syntax for NSNumbers:

NSDictionary *data = @{
                      // when you always pass same value
                      @"someKey" : @YES
                      // if you want to pass some boolean variable
                      @"anotherKey" : @(someVariable)
                      };

Try this:

NSMutableDictionary *dic = [[NSMutableDictionary alloc] init];
[dic setObject:[NSNumber numberWithBool:TRUE]  forKey:@"Pratik"];
[dic setObject:[NSNumber numberWithBool:FALSE] forKey:@"Sachin"];

if ([dic[@"Pratik"] boolValue])
{
    NSLog(@"Boolean is TRUE for 'Pratik'");
}
else
{
    NSLog(@"Boolean is FALSE for 'Pratik'");
}

if ([dic[@"Sachin"] boolValue])
{
    NSLog(@"Boolean is TRUE for 'Sachin'");
}
else
{
    NSLog(@"Boolean is FALSE for 'Sachin'");
}

The output will be as following:

Boolean is TRUE for 'Pratik'

Boolean is FALSE for 'Sachin'