John Anthony Llewellyn (22 April 1933 – 2 July 2013) was a Welsh-born American chemist, a former NASA astronaut candidate, and a NOAA aquanaut.

[1][2] After the award of his doctorate, Llewellyn moved to Ottawa, Ontario, Canada, as a post-doctoral fellow at the National Research Council.

In 1960, he went to Florida State University as a research associate in the Chemistry Department and was subsequently appointed assistant professor.

Llewellyn's diving gave him experience in the feeling of weightlessness, which helped prepare him for his later training as an astronaut.

[6] His then current work was presented at the 2010 American Society for Gene and Cell Therapy and he was an invited session leader at the 2010 Gordon Conference in Bioelectrochemistry.