Yes. The Catholic would have to obtain permission from his or her local ordinary to marry a non-Christian (or, to marry a non-Catholic if the atheist had been baptized before turning to atheism), but the couple could be married in a Catholic church. Whether it would be wise for a Catholic to marry someone who does not believe in God is an entirely separate question. While the Church allows Catholics to marry outside the faith when they are determined to do so, the Church strongly discourages Catholics from marrying outside the faith partly because such marriages make it difficult for a Catholic to meet the duty to raise children within the Catholic faith. Meeting such an obligation would be especially difficult when the non-Catholic spouse is an atheist.
