The Isfahan Observatory (Persian: رصدخانه اصفهان) or Malikshah Observatory (رصدخانه ملکشاه) was an astronomical observatory constructed during the reign of Sultan Malik Shah I by Omar Khayyam, closing shortly after his death in 1092. It was from the work at the observatory that the Jalali Calendar was adopted. Aside from the work on the calendar, little is known about the purpose of the observatory, as no comprehensive sources from its time period are known to exist about the work performed there.