The mission, scheduled for launch in August 2011, would reach the largest planet in our solar system in 2016.
The spacecraft is being assembled in a high-bay clean room at Lockheed Martin Space Systems in Denver.
"We're excited the puzzle pieces are coming together," said Scott Bolton of the Southwest Research Institute in San Antonio, the project's principal investigator. "We're one important step closer to getting to Jupiter."
NASA said Juno will have nine science instruments on board to investigate the existence of a solid planetary core, map Jupiter's intense magnetic field, measure the amount of water and ammonia in its atmosphere and observe the planet's auroras.
"We plan to be doing a lot of testing in the next few months," said Jan Chodas, the project manager based at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "We want to make sure the spacecraft is ready for the long journey to Jupiter and the harsh environment it will encounter there."
NASA said JPL manages the Juno mission, with the Italian Space Agency contributing an infrared spectrometer instrument and a portion of the spacecraft's radio science experiment.