As the Black and Yellows announced on Monday, the 18-year-old Greece international has signed a contract in Dortmund until 30 June 2031. Karetsas had already completed his medical on Wednesday.
"With his creativity and his game intelligence, Kos will enrich our team. He is technically very accomplished and has the ability to find quick solutions in tight spaces, especially in the final third," sporting director Ole Book is quoted as saying.
Karetsas himself was equally upbeat about the next step in his career: "Borussia Dortmund are a special club and I am incredibly proud to now be able to wear the black-and-yellow shirt. I am really looking forward to getting to know my team-mates and soon playing in this impressive stadium in front of our fans. I want to play my part in helping us achieve success together."
BVB transfers: Only Dembele cost more than Karetsas
According to media reports, Borussia are paying a base transfer fee of 30 million euros, plus bonus payments of up to two million euros, to the four-time Belgian champions for Karetsas. According to the Ruhr Nachrichten, Genk also secured a ten per cent sell-on clause in the negotiations.
That means the total package, worth around 32 million euros, would make the attacking midfielder slightly cheaper than Dortmund's previous record signing Ousmane Dembele, who joined the Westphalians from Stade Rennes in the summer of 2016 for 35 million euros.
For Genk, though, the deal is a record sale. Karetsas replaces Tolu Arokodare, who was sold to Wolverhampton Wanderers for 26 million euros last year.
BVB are also said to have reached an agreement with the Greek several weeks ago, but because of stubborn transfer demands they were banging their heads against a wall with the Belgian top-flight club right until the end.
Getty ImagesKonstantinos Karetsas is set to become Julian Brandt's successor at BVB
At the Black and Yellows, Karetsas is being lined up as the successor to Julian Brandt, whose contract at BVB expired at the end of last season. Alongside Dortmund, numerous other clubs had also shown an interest in the teenager, including, according to rumours, AC Milan and Paris Saint-Germain. However, the gifted technician wanted to join only Dortmund.
Born in Genk, Karetsas joined the youth set-up of Belgian top-flight club KRC as a six-year-old. During his academy years, he spent two and a half years at RSC Anderlecht before returning to Genk at the start of 2023.
From there, he made his top-flight debut at the age of 16, and last season he was already a key player for the traditional club from central Belgium. In the completed campaign, Karetsas registered three goals and a remarkable 18 assists.
After Joane Gadou, Kaua Prates and Justin Lerma for a combined total of around 30 million euros, Karetsas is already BVB's fourth new signing this summer. However, Borussia's transfer business apparently is not yet complete: Said El Mala from 1. FC Cologne is also still expected to arrive.