Does salt need a hechsher?

Answer: No (Star-K, cRc).

Reason: Plain salt has no kashrut concern.

Source:

• Star-K: its list of products that need no hechsher includes salt, provided there are no additives and it is not a product of Israel.
• cRc: salt is recommended even without hashgachah, including iodized and sea salt.

Related questions:

• Does sugar need a hechsher? No (Star-K, cRc).
• Do spices need a hechsher? Single plain spices: no (cRc).

References: Star-K, “Approved Without A Hechsher” list · cRc (askcrc.org), “Salt”