Sesame is mainly used for oil. What is left after extracting the oil (sesame meal) is fed to animals. Whole sesame seeds are added to lots of dishes, including bagels, hamburgers and challah. Popular middle-eastern dishes with sesame are tahine (sesame paste) and halva (sweets). Sesame seeds are listed as kitniot on most of the lists you find online (e.g., OU, Kashrut.com, Star-k, cRc Guide).
To understand why sesame is included in the category of kitniot, we must look at some material that is not so directly related to Pesach. The Shulchan Aruch Yoreh Deah 297:3 includes sesame in the category of "kitniot" in a discussion about Kil’ayim, forbidden mixtures. Rambam defines kitniot as "seeds which people eat" in his Hilchot Kilayim 1:8 and also includes sesame. The definition contrasts kitniot with "zaronei gina", vegetables where we eat flesh and seeds (tomatoes, cucumbers, etc.). Although these statements are made way before the minhag of kitniot started and have no relation to Pesach, it is possible that whenever later authorities refer to "kitniot", they refer to the same category. This would make sesame seeds kitniot beyond any doubt. Interestingly, there still seems to be quite some confusion about the kitniot status of sesame seeds, leading right up to kosher lePesach sesame candy (Shira: Open, sesame! Or not. A Pesach question).
Botanically, sesame is a flower from the Pedaliaceae (or sesame) family. Basically, it means sesame is related to a bunch of flowers, none of which you are likely to know. If we go up to the level of order (it is Lamiales), Wikipedia lists lavender, mint and rosemary as members of the same order. But order is already a very broad category! For comparison, beans are in the order of Fabales – as are roses. So let’s just say sesame is not related to anything relevant for the kitniot discussion.
In the raw form, sesame seeds are much smaller than grain, so there is no real confusion possibility. There is sesame flour (apparently there is flour made out of anything), but it’s not something you use daily. There is a ruling by Rabbi Kook (Orach Mishpat 108-114), where he permits sesame oil "since not only are the seeds not malted, but the oil is also fried, which would prevent cereal grain from becoming chametz and is certainly enough to alleviate the problem of kitniyot." (Peninei Halacha)
- Danger of chametz traces: Don’t know.
- Danger of confusion with chametz (raw): No.
- Danger of confusion with chametz (processed): No.
- Botanical categorization: Pedaliaceae (flower)
- Known in 13th century: Yes.
- Verdict: Probably yes.
photo (c) Sanjay ach