Does cereal need a hechsher?

Answer: Yes (Star-K, OU).

Reason: Even cereal can be non-kosher, e.g. from raisins coated with animal-based glycerin.

Source:

• Star-K: its cereal list says the “label must bear a reliable kosher symbol.”
• OU: a cereal may be non-kosher because it has raisins coated with a non-kosher, animal-based glycerin.

Related questions:

• Do oats need a hechsher? No, if plain (Star-K, cRc).
• Do potato chips need a hechsher? Yes (OU).

References: Star-K, “Cereals and their Brachos” list · OU, “What Is Kosher”