Qays ʿAylān (Arabic: قيس عيلان), often referred to simply as Qays (Kais or Ḳays) were an Arab tribal confederation that branched from the Mudar group. The tribe does not appear to have functioned as a unit in the pre-Islamic era (pre-630). However, by the early Umayyad period (661-750), its constituent tribes consolidated into one of the main tribo-political factions of the caliphate.

The major constituent tribes or tribal groupings of the Qays were the Ghatafan, Hawazin, Amir, Thaqif, Sulaym, Ghani, Bahila and Muharib. Many of these tribes or their clans migrated from the Arabian Peninsula and established themselves in Jund Qinnasrin (military district of northern Syria) and the Jazira (Upper Mesopotamia), which long became their abode. From there they governed on behalf of the caliphs or rebelled against them. The power of the Qays as a unified group diminished with the rise of the Abbasid Caliphate which did not derive its military strength solely from the Arab tribes. Nonetheless, individual Qaysi tribes remained a potent force and some migrated to North Africa and Iberia where they carved out their own power.