Rho (upper case Ρ, lower case ρ) is the 17th letter of the Greek alphabet. In the system of Greek numerals it has a value of 100. It is derived from Semitic Rêš "head" (see Resh).

Rho is generally classed as a liquid (together with lambda and sometimes the nasals mu and nu), which has important implications for morphology. It is pronounced similarly to the letter r in languages with a Latin-derived alphabet. In polytonic orthography a rho at the beginning of a word is conventionally written with a rough breathing (equivalent to h) and a double rho within a word is written with a smooth breathing over the first rho and a rough breathing over the second, apparently reflecting an aspirated or voiceless pronunciation in Ancient Greek, hence the various Greek-derived English words which start with rh or contain rrh. The name of the letter is written in Greek as ῥῶ (polytonic) or ρω (monotonic).

The lower-case rho ρ is used to designate the rank and reshape operators in the APL programming language.

Letters that arose from rho include Roman R and Cyrillic Er.

Lower case ρ is used in physics to represent density and resistivity, and in economics to represent the discount rate of future cash flows. In such occasions, it is often represented as ϱ, to avoid confusion with the Latin letter p.