Final answer:
Rollo May was the principal American spokesperson who introduced European existential thought to American psychotherapy, moving it away from Freud's focus on the unconscious.
Step-by-step explanation:
The person who was the main American spokesperson of European existential thinking as it is applied to psychotherapy was Rollo May. European existential philosophy, a movement that emphasizes individual existence, freedom, and choice, had a significant impact on various intellectual fields, including psychology. Rollo May, an American psychologist, was particularly influential in bringing these existential concepts into the realm of psychotherapy in the United States. He integrated ideas from European thinkers such as Sartre and Heidegger into his own work on the human condition, anxiety, and the will. His contributions helped to cultivate a branch of psychology that focused on the human experience and the development of personal potential, marking a departure from the psychoanalytic focus on unconscious impulses and childhood experiences as championed by Freud.