Felix Leopold Oswald (December 6, 1845 – September 27, 1906) was a Belgian American physician, naturalist, secularist and freethought writer.

[1] In 1866, as a military doctor he joined a corps of Belgian volunteers in support of Emperor Maximilian I of Mexico.

He urged a legislative act to protect "the woods of all the upper ridges in hill countries.

[2] Oswald was supportive of natural hygiene, a movement which advocated fasting, vegetarian dieting, pure water, clean air and exercise.

He argued that "the resemblances alleged by Dr. Oswald, even if granted, would be insufficient to prove his case... the differences between the Gospel and Buddhism run deeper and are more positive than the like-nesses.

Orientalist Friedrich Max Müller rejected Oswald's thesis but respected his dedication to the subject.

[2][19] Obituaries have described it as a tragic accident,[11] whilst railway employees reported that he had committed suicide.