The Advertising Standards Authority (ASA) received a single complaint that the ad (below, right) was "irresponsible and offensive" due to the size of the female model.
Calvin Klein said it was "long established" in the apparel industry for swimwear ads to feature young, toned models.
They also provided a letter from the model’s agent claiming that she had a suitable Body Mass Index.
The ASA agreed that the format of the photograph "emphasised" her "slim and toned physique", but disagreed that she appeared underweight. It concluded that the ad was not irresponsible and could be used again.
Brands have been criticised in the past over the use of "size zero" models. In 2001, Topshop removed an image of a size zero model from its homepage following negative media attention.
Earlier this year, Debenhams banned the use of airbrushing in its ads, arguing consumers should not be "bombarded" with "unattainable body images".