Dikastes (Greek: δικαστής, pl. δικασταί dikastai) was a legal office in ancient Greece. In the broadest sense, a dikastes was a judge or juror. However, the term usually refers more specifically to a judicial officer in ancient Athens during its democratic period who formed part of a body constitutionally empowered to judge matters of law and custom.

The Athenian dikasterion (court) resembled the modern Western jury in some respects. For instance, both consisted of a number of members of the public, assembled in the name of the country to judge between its individual citizens, who took office after swearing an oath to well and truly discharge their duties. However, there were otherwise significant differences between the two types of institutions.

In Athens, 6,000 people were selected by lot as dikastai for each year. To be eligible to serve as a dikastes, an individual had to be:

Selection was supervised by the Athenian archons and their official scribe, each of whom drew the names of six hundred members of their phyle. The 6,000 drawn were then sorted, again randomly, into eleven sections: 10 sections of 500 members each, and a supernumerary section of 1,000 to supply individuals in the event that members of the sections of 500 were unavailable. Each of the 10 sections was designated with one of the first 10 letters of the alphabet.

When the dikastai had been selected and assigned to sections, each dikastes was provided with a small tablet called a pinakion (πινάκιον; pl. pinakia), which served as a certificate of their appointment. Dikastic pinakia bore the letter of the section and the name of the individual. A number of bronze plates have been found in Piraeus which are believed to have been dikastic pinakia; they bore the information described above, as well as owls, Gorgon heads, and other heraldic devices associated with Athens. The supernumerary dikastai may have had a different token, but the evidence in this regard is inconclusive.

Before assuming his office, the dikastes had to swear the oath of office. This was initially done on the Ardittos Hill, located just outside ancient Athens and a short distance east of the Ilissos River. At later times, however, the place of oath-swearing moved to another venue which is unidentified. In Demosthenes' time, the oath included the following elements:

Once the dikastai were selected and assigned, and had sworn their oaths, their appointment lasted for a year. During that year, each time it became necessary to empanel a number of dikastai to try a case, the thesmothetai (junior archons) would carry out the process for doing so.

One or more sections might be assigned to try a case. In most cases, court assignments were decided by lot: tickets would be drawn from two containers, one containing tickets signifying individual sections of dikastai, the other containing tickets denoting courts. For a case which required a single section of dikastai, the thesmothetai would draw one ticket from each container, determining that, for instance, section B would sit in court F. If the case required multiple sections of dikastai, the thesmothetai would carry out essentially the same process, but draw as many additional tickets as required from the "sections" container, determining that, for instance, sections A and B would sit in court F. When court assignments had been made, each dikastes received a staff (βακτήρια or ῥάβδος) bearing the letter and colour of the court to which he was assigned, which authorised him to enter the court and distinguished him from members of the public attempting to do the same.