Meursault could certainly be considered an existentialist figure, though it would be more accurate to term him an absurdist one. Existentialism is a complicated topic, involving thinkers as diverse as Søren Kierkegaard and Jean-Paul Sartre, but defined broadly, it essentially states that the individual is responsible for providing life with meaning. This is done in the face of an inherently meaningless reality. Existentialists can be religious or irreligious, but all agree that whatever meaning life has must be provided by the individual, not by any outside force.
Although he is often associated with the French existentialists, Albert Camus himself rejected this label. His own philosophy of absurdism differs from existentialism in its emphasis on the idea of embracing the absurd, or life's meaninglessness, rather than constantly striving to create and maintain a sense of meaning.
Meursault, the narrator of Camus's novel The Stranger, certainly views reality as absurd and meaningless, even irrational. He rejects philosophies that try to graft order and meaning onto reality, such as the devout Christianity of the magistrate. He offers no rational explanation for the killing of the Arab. He places no value on social institutions such as marriage or familial loyalty. Instead, the amoral Meursault responds only to the physical world. He enjoys smoking, sex, swimming, and other such comforts. They are about the only things to which he ascribes any value.
While the reader might find Meursault a rather cold and borderline sociopathic person, Camus's point was to make Meursault an absurdist figure, someone who navigates an absurd universe according to his own perspective and not the one society desires everyone to share. In the end, acceptance of this perspective grants Meursault true inner freedom, even as he is about to die. This is the freedom Camus believed could be achieved by accepting the absurdity of existence and the inevitability of death.