Rules of Golf Review: You're playing a Titleist. Two holes later, it's a Callaway. Is switching golf balls mid-round allowed?
For many golfers (us included), finishing a round with the same golf ball you started with is a real source of pride. It might look like it was chewed on by a goat for a few hours, but hey, if you don't lose it, at least you can say you kept it in play all afternoon.
For those, however, who can’t help but lose a ball or three during a round (us included), a natural question emerges: If you start playing your round with one type of golf ball, do you have to keep playing the same type of ball the rest of the round? For example, if you played a Titleist ProV1 on the first hole, do you have to commit to that brand—and that model—until you hole out on 18?
The answer is: Sometimes.
There is an option in the Rules of Golf for a course/committee to employ something known as the “one-ball rule.” Technically, this is Model Local Rule that can be put in effect—almost always only in competitions—that says you must play with the same brand, make and model of ball you started the round with. This means that if you start playing with a Srixon Z-Star, you must play that type of ball for the remainder of the round and may not switch to another brand or even another model of Srixon. For reference, see "Committee Procedures; Model Local Rule G-4." The penalty for somebody who mistakenly plays a different ball is two shots for each stroke made with the incorrect ball.
The one-ball rule is in effect on the pro tours, and it does come into play every now and then. Back in 2019, Russell Henley was penalized eight shots (seriously) for switching between two different types of Titleist ProV1s—unknowlingly—for four holes at the PGA Tour's Maykoba Golf Classic.
Keep in mind that for the majority of your rounds, switching balls is fine between holes. You also can substitute a ball during play of a hole if you are taking relief (free or penalty), or if the ball is cut or cracked and the damage happened during play of that hole. You can't switch out a ball if it's simply scratched, scraped or the paint is damaged (Rule 4.2).