It's now been relieved of its transport container and raised into the vertical to allow preflight checks to begin.
JWST is one of the grand scientific projects of the 21st Century and will ride to orbit on 18 December.
An Ariane-5 rocket will throw the telescope out to an observing position about 1.5 million km from Earth.
From there, it will look deeper into the cosmos - and thus further back in time - than is possible with Hubble.
It will do this with a much bigger mirror (6.5m in diameter versus 2.4m) and instruments that are tuned to the infrared.
Scientists hope this set-up can detect the light from the very first population of stars in the Universe to switch on more than 13.5 billion years ago.
JWST is a joint venture between the US (Nasa), European (Esa) and Canadian space agencies (CSA).
It's taken more than three decades from the original conception to get to this point.
Final assembly and testing was completed in August at the Northrop Grumman factory in Redondo Beach, California, after which the telescope was made ready for a 16-day, 2,500km journey by sea to French Guiana, a trip that took the observatory through the Panama Canal.
Teams at Europe's spaceport will first inspect JWST to confirm no damage was picked up in transit. The telescope will then be fuelled and mated to the Ariane 5.
Recent weeks have seen cargo planes arrive in French Guiana with the tools and support equipment needed to work on Webb over the coming weeks.
A key milestone in the preparations comes this Friday when another Ariane-5 is due to launch two communications satellites from Kourou. This has to take place to free up the launch table on which Webb's rocket will be integrated.
Article Source: https://www.bbc.com/news/science-environment-58960575
