What do you mean by that? There are new volcanic islands being created reasonably frequently, so by some definition the answer is "a small island off the coast of Japan, which appeared a few months ago and was discovered shortly after it came into existence".

If you mean major landmass, then mainland Antarctica was first observed by Westerners in 1820. It's likely that they were the first humans to see it and return to tell anybody else about the experience. There are some suggestions that Māori sailors may have ventured far enough south to encounter Antarctic sea ice, but I don't believe anybody seriously suggests they reached the mainland.