An industrial team led from EADS Astrium in Germany has completed the build of the Near-Infrared spectrometer, one of four instruments that will go in JWST.
NirSpec's job will be to determine the age, composition, movement and distance of the objects in its field of view.
The expectation is that some of these targets will include the very first stars to shine in the Universe.
That would mean picking up light signals that have travelled across space for perhaps 13.6 billion light-years - something Hubble cannot do.
JWST will make it possible with a suite of next-generation technologies, including a 6.5m primary mirror (more than double the width of Hubble's main mirror), and a shield the size of a tennis court to guard its keen vision against the light and heat from the Sun.
NirSpec is critical to this new capability, and represents 10 years of design and manufacturing endeavour.
In a short ceremony in Ottobrunn on Friday, the instrument was handed over to the European Space Agency (ESA), which had commissioned NirSpec.
The Paris-based organisation then immediately passed the near-200m-euro instrument to the US space agency (NASA), which leads the JWST venture.
On 20 September, NirSpec will be flown to Maryland's Goddard Space Flight Center for integration into the giant orbiting observatory.
Europe's major industrial commitments to JWST are now complete.
Its other instrument - the Mid-Infrared Instrument (Miri), which was assembled in the UK - was safely delivered to North America last year.
The one outstanding task - and it is a very onerous one - will be to launch JWST in October 2018.
This will be performed by an Ariane 5 rocket from Esa's Kourou spaceport in French Guiana.
When I visited NirSpec in the Ottobrunn clean room last week, there was not much to see because the finished instrument was dressed for shipment in its protective thermal coat.
But if you could lift that covering, you would lay eyes on what appears to be an impossible optical maze.
NirSpec will be mounted just behind JWST's primary mirror and will sample the gathered light via a kind of periscope.
A series of mini-mirrors will then corral and condition this light, moving it towards a grating element where it can be sliced and diced into its component colours - its spectra.
Detectors are positioned at the end of the maze to read these colours and convert them into an electronic signal that can be transmitted to the ground.
All this is done in the near-infrared, in the wavelengths from 0.6 to 5 microns. This is the region of the electromagnetic spectrum where you would expect to pick up starlight that has been stretched on its 13-billion-year journey across an expanding cosmos.
An interesting aspect of NirSpec's design is that nearly half by weight of the instrument is made from ultra-stiff silicon carbide.
"The unique feature of silicon carbide is that it allows us to make structure and mirrors out of the same material," explains Astrium programme manager Ralf Maurer.
"This helps us survive the transition going from warm to cold; there is no deformation. And that gives us a very stable alignment of the optics."
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