Short answer: quite possibly, but we unfortunately don't know any real details about who he was or what he did.

I wrote a longer answer to a similar question a few days ago. You can follow the link to see the whole discussion, or read the most relevant bits here:

The problem is that we actually have very little contemporary evidence about Arthur, and the stories that do survive are not only written much later, but are clearly entirely works of fiction. That doesn't, however, make it entirely impossible to talk about the world the original Arthur (if he existed) would have lived in.

There's a tiny bit of evidence for Arthur being a real person. The Y Gododdin (a poem probably written in the late 6th or 7th century) mentions a heroic leader who 'was not Arthur'. Gildas (Welsh, writing in the late 5th or more probably mid 6th century) mentions the battle of Badon Hill (but not Arthur himself). But that's about it, and these brief mentions tell us no information at all about who this Arthur was or what he did.

For more details, you have to go to sources that are written much later, and these sources have problems that make it difficult to trust the stories they tell. A Welsh chronicler named Nennius gives a bit more information about Arthur in an account probably written in the 9th century, but this account is suspect for two reasons. The first is that it contains a lot of other stories which are rather mythical (for example, he also talks about a 300-year-old holy man). It's not clear that Nennius shared our modern distinctions between 'fact' and 'fiction'. It's also clear (secondly) that his text was written during a time of conflict between Nennius' king and a neighboring Anglo-Saxon kingdom. His Chronicle often reads like wartime propoganda, and the stories it tells about Arthur seem to be pretty heavily spun to make Arthur out to be the folk hero the 9th century Welsh needed to help them beat their contemporary Anglo-Saxon neighbors. This makes it very difficult to separate out the contemporary politics from whatever source material Nennius may have been drawing on; was Arthur originally an enemy of the Saxons? Or is that just Nennius' politics coloring his history? Plus, Nennius actually doesn't give us much detail about Arthur - just a list of battles Arthur supposedly won.

For the good, detailed accounts about Arthur (the Round Table, etc), you have to look to the later middle ages. Geoffrey of Monmouth, writing for Norman royalty in the 12th century (remember: Arthur was probably alive in the 5th or 6th century), used Arthur as a great example of how Britain's best kings were enemies of the Saxons (just like the Norman conquerors were, what a coincidence! ie, this is also politics masquerading as 'factual' history). He claimed to be basing the detailed stories he told about Arthur on lost Welsh histories that he had found... but who knows. Odds are good that he invented the details or, at best, borrowed a bunch of fictional welsh folk stories.

After Monmouth, stories about Arthur explode in popularity, and we get all the details we know so well. Most of our modern ideas about Arthur come from Malory's Morte D'Arthur. But at this point, we've left history far behind, and are dealing with Arthur the literary hero - Malory had about 200 years of late medieval stories to draw on for his version of events, and was writing nearly 1000 years after the real Arthur would have lived, if he were a real person at all.

I'd recommend G. Halsall's Worlds of Arthur if you want to really dig into the early medieval evidence. It's readable, inexpensive (though ILL is your friend, always), and gives a great introduction to how historians and archaeologists study the early middle ages. Halsall concludes that Arthur may well have existed, but that we simply don't have the evidence to say any more than that about him. I think that's a correct, if disappointing, conclusion.

(continued)

The following is an answer I developed to these sorts of questions in general (and you will find the question of Arthur embedded in the midst of the discussion). The question you raise is so similar to others that are asked in a variety of venues that I included this answer in my Introduction to Folklore. I hope you find this useful:

When I see the posts like this asking about whether there were real people or events behind legends, myths, and/or the ancient gods, I respond with a number of observations. First, the idea that the gods and heroes of legend are based on real people had an early proponent in the Greek, late-fourth-century BCE writer, Euhemerus, giving his name to this approach to myth and legend: Euhemerism. Folklorists generally regard the idea that there was an actual basis for most oral tradition as barking up the wrong tree, because the original “real” event behind a story is usually elusive and searching for that core is a futile exercise. In addition, research into how stories began usually concludes that they emerge in a rather spontaneous way, typically without an actual incident to inspire them.

A few examples: the Classical Greek story of Perseus is an early manifestation of a widespread folktale, catalogued by the twentieth-century folklorists Antti Aarne and Stith Thompson as AT 300, “The Dragon Slayer.” Was there a proto-Perseus who rescued a maiden from some sort of extraordinary threat or perhaps from some sort of human sacrifice? It is hard to answer that question, but it is not hard to imagine how far back in time that proto-incident would have had to occur: AT 300 is spread throughout Eurasia. It was collected from cultures that could have no conceivable literary connection with ancient Greece, and yet the shared assortment of motifs in the numerous variants clearly show some sort of genetic, that is, historically-connected relationship. Would we need, therefore, to go back thousands of years before the first recordation of the Perseus story in order to find this proto-Perseus? It is much easier to understand that the folktale simply diffused and that one of its manifestations was in ancient Greece.

Now, let’s consider another example that has inspired a lot of spilled ink. A simple Google search for the “origins of King Arthur” provides more websites than one could easily read in a week. Was there a proto-Arthur? Perhaps. Maybe there were several. But what does that prove? Every society has remarkable characters, and it may be a natural process for these sorts of individuals to attract all manner of traditional stories that have nothing to do with the original inspiration of the cycle of legends.

So what do we have with Arthur? Was there a core source (or sources) for this legendary character? Let’s concede for the sake of argument that the answer is yes. Now, did this individual have a great warrior at his side who became ensnared by the leader’s wife in the fashion of Lancelot and Guinevere? That is more problematic since this type of story is also associated with Diarmuid and Grainne in the Irish court of King Finn and with the Cornish stories of Tristan and Isolde in the court of King Mark. One could even argue that it is the story behind Helen of Troy. In fact, it appears that this was a widespread type of story that became associated with various courts of historical legend. We cannot conclude that every great king had a queen who was attracted to one of his warriors and coerced him to take her away. This is simply a story that was attached to cycles involving great courts. In short, the further one goes back to find the “real Arthur,” the less the candidate (or candidates) look like the King Arthur who has been beloved for centuries. The proto Arthurs are not really King Arthur. They may be seeds but they look nothing like the tree that would grow over the centuries. We do not hold an acorn and say “Ah, I have in my hand a mighty oak tree.” It is not yet a tree. It is a seed. And the two look very different even if they are genetically linked.

One more example: there is a widespread legend told by countless families of the ghostly appearance of a loved one in anticipation of news that the individual died. This became a popular tradition in post-Famine Ireland because so many relatives lived in North America or elsewhere. But it is frequently told by all sorts of people internationally. So we can ask, are there real-life, actual inspirations for this legend? That is, do the spirits of the dead actually come to visit loved ones? Well, how the hell should I know? To paraphrase a famous line from the television show “Star Trek,” “Damn it Jim, I’m a folklorist, not a ghost hunter.” And I have no intention of becoming a ghost hunter. It doesn’t matter what is behind stories so much as it does that people tell these stories. I’m in it for that part of the game; I consider stories as they are told over time, to gain from that material some insight into the past, into culture, and into the human condition. I am a folklorist. And with that, my plate is full.

There's a book on this exact subject by Guy Halsall called "Worlds of Arthur". I haven't read the whole thing, but his consensus is that the modern figure of King Arthur came from a variety of historical figures and legends.