Is Superman vulnerable to magic?

In the series Smallville, Superman is weak to any form of magic, or at least doesn't have any kind of protection against it. Just as with anyone who is heavily protected, a chink in the armor can prove fatal.

There's an excelent article about this very topic as well on How Stuff Works. Basically, it pits Superman vs Harry Potter and Gandalf, to which they suppose he would lose both battles.

In Kingdom Come when Wonder Woman and Superman are in space, Superman cuts himself on her sword. Wonder Woman warns him to be careful as he is weak to magic and the sword was forged by the gods.

Since KC Superman was identical to normal universe superman until the divergence point, then the answer is clearly yes.

This is further expanded on in World War 3 when Black Adam goes crazy, and Superman states he isn't a match for him.

Those are the most prominent examples that comes to mind.

No. Superman is no more vulnerable to magic than any other person in the DC Universe. But given his range of powers and normal ranges of invulnerability, this is counted as a great weakness relatively speaking. While magic-users do not abound in the DC Universe, there are a number of magical phenomenon which can make confrontations against those forces a surprise for the Man of Steel.

Superman is indeed vulnerable to only a tiny slice of phenomenon in the DC Universe, and each iteration was slightly different in their representation of those weaknesses. We will use the Golden Age Superman, Pre-Crisis Superman and Post Crisis Superman as the definitive canon models as reference. DCnU Superman's limits are still being defined, but it is likely they will not deviate too far from the norm.

All canon Supermen:

• Derived their powers from the yellow or higher energy wavelength stars. Orange or red stars do not completely trigger the genetic capabilities that give him his powers. Under orange stars, his powers are half of their strength and under red stars he has no access to his superhuman abilities.
• While this has never been effectively explained, Kryptonite appears to possess the ability to prevent him from accessing the solar energy somehow stored within his cells. In addition to denying him access to his solar powers, prolonged exposure is both dangerous and ultimately toxic to him. Upon the removal of Kryptonite, depending on the exposure, his access to his powers will return relatively shortly. Only a near-death encounter will leave him drained of power.
• possess a weakness to specific wavelengths of radiation. Advanced species who knew of Kryptonians (such as the denizens of the Fourth World, i.e. Darkseid, Highfather and other New Gods, as well as extraterrestrials, particularly during the period of Pre-Crisis Superman who spent a considerable amount of his time in space) used weapons capable of producing radion energy. Such weapons were rare but were capable of affecting Superman.
• Pre-Crisis Superman was also vulnerable to exotic radiations such as Q-radiation and weapons from advanced civilizations that either harnessed red sun wavelengths, or with sufficient energy could overcome his invulnerability making him vulnerable to their effects. Green Lanterns (and presumably the Guardians) can also use their rings to emulate the wavelengths of Kryptonite radiation.
• have less of a weakness and simply an inability to effectively counter magical energies. The true magic of the DCU has always been considered an effect of Chaos and derived from the Lords of Chaos. Such effects of Chaos render a being whose powers are derived from Order less than effective. Superman is not an agent of Order, only that his powers are created from a technological process that is Ordered.
• have a vulnerability to magic that is no greater than any ordinary human's and Superman has been able to fight magical foes, granted with more difficulty, but his overall effectiveness is on par with any of his other villains. One supposition used during the Byrne era's Superman was that magic bypasses his ability to defend against it due to his inability to understand it. Other suppositions assumed his alien heritage left him vulnerable but who can be sure? As usual, DC has never effectively explained why magic works against him when so many other things do not.

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Inconsistent storytelling with poorly defined mechanics

Given that magic is not explained effectively, it can be inferred that metahumans such as Black Adam and Captain Marvel who derive their powers from a magical source are more effective against Superman and that would be a misnomer. The strength of Hercules used by Captain Marvel is super-strength on par with Superman's.

His ability to affect Superman is because his strength is as great as Kal-el's not because it is an active magical effect. Unless the Captain or Black Adam are using a directed energy attack such as the Summoning Lightning Discharge against Superman, or a directed magical energy attack, it is a struggle of physical equals. However, the summoning lightning is a powerful directed magical attack and has been used against Superman to stun and weaken him.

Magical objects, weapons and the like are able to affect him and bypass his natural defenses. Hence Wonder Woman's weapons, forged by powerful magical divine beings, are more than capable of harming Superman. He has also been scratched by werewolves and vampires whose magical nature is inherent and required for their very existence.

In most storylines, Superman has been considered to be equal to or slightly ahead of Captain Marvel in terms of raw power and durability. Superman has always been considered to be the DCU flagship hero on Earth in terms of physical superiority with Captain Marvel being considered second to him and titled "the World's Mightiest Mortal". Given that DC prefers to have the two characters locked in a perpetual dead heat because it promotes fanboy enthusiasm, it will likely never be revealed which of them is more powerful.

A note: All of the references used as examples in other answers are non-canon references, Superman/Shazam!: The Return of Black Adam, Smallville and Kingdom Come are all divergent timelines/continuities and do not reflect canon Superman. Nor should they be used as definitive references to the character.

Superman has indeed been shown to be vunerable to magic, at least in some universe versions For example, when he encountered Arathaza (New Earth, written/illustrated by John Byrne),

I think it's worth creating a sort of addendum for the all the answers here that it's not so much that Superman is weak against magic as it is that he is not specially strong against it. This is something other answers hinted at, but never outright stated.

I believe there is a subtle difference there, in that it does still allow Superman to do some things vs magic users and weapons that you or I would be unable to do. For example, a weapon that is magically enhanced to hit harder (say, 10x the force) will hit Superman harder. Superman is not immune to magic, and therefore Superman cannot negate the weapon's effect. He still has to deal with the 10x force blow. At the same time, the weapon does not get the ability to punch through Superman's defense and hurt him as if he were human. So it's not as if you or I took a punch with 10x the normal power. Because Superman still has his great durability, Superman may take the shot and keep fighting.

Old topic but yes, he is vulnerable to magic.

People say he is no more vulnerable than any other person in the DC universe.. but take that in context.

So Superman has the same defense vs it as a normal unpowered person. Magic is supposed to be a trump card Vs superman because getting a red sun or Kryptonite has not been an easy thing.

All superheroes have a weakness or they would be gods and even gods in Marvel and DC have weaknesses.

The only thing that can save Superman and lessen the impact is once he is aware of what he is facing he can use super speed to avoid most attacks or impacts.

Wonder Woman and the Lasso would own him. Dr Fate or Dr Strange (assuming a crossover) .. would own him. The issue is magic has to hit him so unless it's an area effect.. once known, his speed should compensate.

Superman also holds back and is arrogant. Again if this was not true.. almost nothing would be a fight for him.