In a rather terse tweet, the Yorkshire club bade farewell to the 20-year-old, who joined their academy in 2015.
Since signing his first professional contract with Leeds in 2016, Vieira has made 60 appearances in the Championship, scoring one goal. He was shortlisted for the club's Young Player of the Year award in 2018, though it was ultimately won by Bailey Peacock-Farrell.
Vieira is a defensive midfielder, so Sampdoria may regard him as a natural replacement for Uruguay international Lucas Torreira, who has moved to Arsenal.
Admittedly, it may take the 20-year-old some time to adjust to the greater technical demands of Serie A, after two seasons in the second tier of English football. However, Sampdoria only finished in tenth place in the Italian top flight last season, so expectations are unlikely to be excessively high.
In addition, Vieira has proved that he can handle the pressure of international football, albeit at youth level. Although he is eligible to represent Guinea-Bissau (his country of birth) and Portugal, he has featured eight times for England's Under 20 side and three times for the Under 21 side - winning the Toulon Tournament twice.
Meanwhile, Leeds fans are unlikely to be delighted with the loss of one of their most promising young stars, especially in the wake of their club's failure to sign Derby County striker Matej Vydra. On the plus side, the Peacocks have now signed striker Patrick Bamford, who scored ten league goals for Middlesbrough last season.
In any case, Leeds will hope that next season is more successful than last - they finished in a desperately disappointing 13th place in the Championship, 15 points off the playoff places.