To the east of Chandra Mahal lies the observatory also called Jantar Mantar built by the astronomer King Sawai Jai Singh. It is the largest of the five open-air observatories built by him. Built before the establishment of Jaipur, this observatory is made of lime and stone. This observatory is a miraculous example of the geometrical, trigonmetrical and mathematical architecture. It was built in 1718 whereas Jaipur was founded in 1727. Other four observatories were built at Delhi, Mathura, Ujjain and Varanasi. The observatory at Jaipur is the largest and best preserved after the restoration work in 1901. This observatory is very famous because it is based on scientific principles. It is an exquisite example of the Architectural art.

The importance of this observatory continues even today. From time to time famous astrologers and scholars come here to measure local time, the altitudes of heavenly bodies, the sun's declination, meridian altitude and azimuth and determine eclipses with the help of various instruments that exist here. People are amazed to see discs, large clocks and narrow lanes and stair cases and are forced to think how in today's world which has telescope and other modern equipment, the stony structures of Jantar Mantar could calculate the position of planets. But it is true that Jantar-Mantar has its importance even in this scientific age. Here the main instruments are Samrat Yantra, Jai Prakash Yantra and Ram Yantra which comprises Sun Dial, Dhruv Naal, Naari Valay, Kranti Vrat, Samrat, Rashi Naari, Valay Rashi Chakra, Jai Prakash Vakra, Kapali, Ram, Digansh and Kranti Vrat Yantra.