Opus 100 is a collection by American writer and scientist Isaac Asimov.

It was published by Houghton Mifflin on 16 October 1969.

Asimov chose to celebrate the publication of his hundredth book by writing about his previous 99 books, including excerpts from short stories and novels, as well as nonfiction articles and books.

Opus 100 also includes five complete science fiction stories and one complete science essay.

Humor Appendix: My Hundred Books Algis Budrys gave Opus 100 a mixed review, saying "the book as a whole demands so many shifts of personality and attitude in the reader that no one, not even Asimov, could keep up with them comfortably.