Zoilus I Dikaios (Greek: Ζωΐλος Α΄ ὁ Δίκαιος; epithet means "the Just") was an Indo-Greek king who ruled in Afghanistan and Pakistan and occupied the areas of the Paropamisade and Arachosia previously held by Menander I. He may have belonged to the dynasty of Euthydemus I.
Zoilos used to be dated after the death of Menander, c. 130–120 BCE (Bopearachchi). Two coins of Zoilus I were however overstruck by Menander I  so Zoilos came to power while Menander was still alive and was perhaps his enemy. R. C. Senior has suggested some time between 150–135 BCE.
Zoilos I uses a silver coin type similar to that of Euthydemus II, son of Demetrius: Crowned Herakles standing, holding a wreath or diadem in his right hand, and a club and the lion skin in his left hand. On some of the coins, which are of lower artistic quality, Herakles is crowned by a small Nike. Zoilos I also struck rare gold-plated silver coins with portrait and Heracles.
The Indian-standard coins of Zoilos I also bear the Pali title "Dhramikasa" ("Follower of the Dharma"), probably related to Buddhism, appearing for the first time on Indo-Greek coinage. A few monolingual Attic tetradrachms of Zoilos I have been found. Zoilos inherited (or took) several monograms from Menander I.
His bronze coins are square and original in that they combine the club of Herakles with a Scythian-type bowcase (for a short recurve bow) inside a victory wreath, suggesting contacts or even an alliance with horse-mounted people originating from the steppes, possibly either the Scythians (future Indo-Scythians), or the Yuezhi who had invaded Greco-Bactria. This bow can be contrasted to the traditional Hellenistic long bow depicted on the coins of the eastern Indo-Greek queen Agathokleia.