Confirmed during a joint press conference held by the leaders of PSA Group and GM, the sale sees the French company leapfrog the Renault Nissan Alliance - which only recently added Mitsubishi to its portfolio - to sit just beneath Volkswagen.
The deal, valued at just over $3 billion (2.2 billion euros), gives PSA a 17 per cent market share in Europe, against the Volkswagen Group's 24 per cent.
While PSA celebrates, the sale brings an end to GM's market presence in Europe, having already confirmed in 2013 it would pull Chevrolet out to - somewhat ironically, now - focus on returning Opel to profitability. Today's sale also marks something of a flip, with GM having once been PSA's second-largest shareholder.
PSA chief Carlos Tavares, credited with bringing the company back from the brink after a French and Chinese bailout in 2014, said today that the loss-making Opel and Vauxhall brands will be made profitable - thanks, in part, to $2.3 billion in joint PSA/Opel cost savings.
“We respect all that Opel/Vauxhall’s talented people have achieved as well as the company’s fine brands and strong heritage. We intend to manage PSA and Opel/Vauxhall capitalizing on their respective brand identities," Tavares said.
"Having already created together winning products for the European market, we know that Opel/Vauxhall is the right partner. We see this as a natural extension of our relationship and are eager to take it to the next level.
"We're confident that the Opel-Vauxhall turnaround will significantly accelerate with our support."
Before today, GM had already confirmed two passenger vehicle lines built on PSA architecture - as part of a deal struck in 2012 - including the upcoming Peugeot 3008-based Opel Grandland X and 2008-based Crossland X. Neither has been confirmed for Australia.
Today's announcement will see PSA license technology rights from GM, so that it can continue to sell existing and already confirmed Opel/Vauxhall models in Europe. Beyond that, in the years ahead, all Opel and Vauxhall models will be built on PSA platforms. The next Opel Corsa is expected to be first PSA-developed model.
Asked if the Opel and Vauxhall brands can be sold successfully alongside Peugeot and Citroen, Tavares said the advantages of each brand is clear.
"The positioning of each is very clear. We need to be consistent, yes. Our brand positioning is clear and we will reinforce what we believe has been a very successful repositioning of Opel by Karl-Thomas Neumann.
"This is a German brand. We have the French brands and now a German brand. And we might not like it, but we have to recognise that some customers want French brands, some want German, some want Asian. It is very important that we have [at least] the French and German brands in our portfolio," Tavares said.
Opel boss Karl-Thomas Neumann touched on the economies-of-scale advantages of joining the PSA Group: "We will have R&D savings, and some 'DNA' gains. In particular, sales and marketing expenses, and we will have much better bargaining power to reduce costs in marketing. [We will have] Manufacturing savings too, in being able to reduce costs in tooling."
Neumann said he expects Opel to be cashflow positive by 2020.
Mary Barra, CEO of General Motors, would not comment on what the deal meant for her company. Instead, she spoke of the "win-win" that today's announcement represents for Opel.
"I think when we look at the Opel/Vauxhall operations, what we saw, was an opportunity to strengthen the business. Going forward, there were changes to customer expectations that made it very clear there was a need for major change to ensure we could keep putting award-winning vehicles on the road. We really think this is a win-win for Opel and Vauxhall," Barra said.
Whether it's a win for Holden, long-term, is unclear. Indeed, the question may go unanswered - officially, at least - for some time.
However, in a statement released today, alongside the PSA and GM announcement, Holden confirmed that we can at least expect the new Opel-sourced Astra and Commodore (Europe's new Insignia) to remain on the Australian market.
"Holden and Opel have had close ties for many years and delivered fantastic vehicles to Australian customers, including the current all-new Astra and the next-generation Commodore due in 2018. The good news is these product programs are not affected at all," the statement reads.
"We will continue to work closely with Opel and GM to deliver our vehicle plans with excellence and precision. This includes future, new right-hand-drive SUVs like the Equinox and Acadia that were engineered specifically for right-hand drive markets."
Likewise, the PSA and GM announcement confirmed "existing supply agreements" for Holden - and fellow Opel beneficiary Buick - will continue.
So, while it is clear that these new GM-developed models will be offered by GM Holden in Australia, their cousins in Europe will be sold, under license, by PSA.
It seems likely, however, that the valuable Astra badge - which will no doubt transfer fully to PSA after the current generation - will again leave the Holden range.
And the next Commodore - if indeed we see another Commodore-badged car after the recently revealed model retires sometime in the early 2020s - will surely be of GM extraction.
There are many questions still to be answered.
For now, watch for the new Commodore to make its Australian debut early next year, just as scheduled.