The obolus (or obol) is a Greek silver coin worth a sixth of a drachma. Its value is approximately three US cents (2003). In Classical Athens it was subdivided into eight chalkoi ("copper pieces").

According to Plutarch, the Spartans had an iron obolus of four chalkoi.

The obolus is also a measurement of weight. In ancient Greece it was defined as one sixth of a drachma, or about 0.5 gram. In ancient Rome it was defined as 1/48 of a Roman ounce, or about 0.57 gram. In modern Greece it is equivalent to one decigram, or 0.1 gram.

The word "obolos" or "obelos" (plural: oboloi, obeloi) means a long thin metal rod, such as a spit. A small "obelos" is an "obelisk", euphemistically. "Oboloi" came to be used as currency because they represented ingots of copper or bronze, and were traded as such. Interestingly enough, Sparta chose to retain the use of the cumbersome, impractical "oboloi" rather than coins proper, so as to discourage the pursuit of wealth.