Koulibaly joined the Blues just last year from Napoli for a reported £33 million ($63 million AUD), but the club has now sold him for roughly half that amount at £17m ($32m).

The 32-year-old struggled in the Premier League as Chelsea finished 12th — their lowest league finish since 1994.

“Kalidou Koulibaly has left Chelsea after completing a permanent transfer to Saudi Professional League side Al-Hilal,” Chelsea said in a statement.

“We would like to thank Kalidou for his contributions on and off the pitch during his time at Stamford Bridge and wish him well for the next stage of his career.”

Backed by the nation’s sovereign wealth fund, the Saudi Pro League is luring big names from the major European leagues with the promise of huge salaries.

Cristiano Ronaldo’s move to Al Nassr in January started the trend. His former Real Madrid teammate Karim Benzema and French World Cup winner N’Golo Kante of Chelsea have recently joined Al-Ittihad.

Koulibaly will join Wolves captain Ruben Neves in moving to Al-Hilal.

Chelsea could be set to further profit from the Saudi investment. Goalkeeper Edouard Mendy is reportedly close to joining Al Ahli, while Moroccan winger Hakim Ziyech is on the brink of a move to Al Nassr.

The English club need to sell players before the end of the month to try and comply with financial fair play regulations after spending over £500 million on new signings over the past 12 months.

Arsenal

Ins:

Outs: Ainsley Maitland-Niles (released), Matt Smith (released)

Aston Villa

Ins: Youri Tielemans (Leicester, free)

Outs: Ashley Young (released), Jed Steer (released)

Bournemouth

Ins: Hamed Traore (Sassuolo, £20m), Justin Kluivert (Roma, undisclosed)

Outs: Jefferson Lerma (Crystal Palace, free), Jack Stacey (Norwich, free), Junior Stanislas (released), Jordan Zemura (Udinese, free)

Brentford

Ins: Mark Flekken (Freinburg, £11m), Kevin Schade (Freiburg, £20m), Ethan Brierley (Rochdale, undisclosed), Romeo Beckham (Inter Miami, free)

Outs: Pontus Jansson (Malmo, free), Tariqe Fosu (released), Saman Ghoddos (released)

Brighton

Ins: Joao Pedro (Watford, £30m), James Milner (Liverpool, free), Mohamed Dahoud (Borussia Dortmund, free)

Outs: Alexis Mac Allister (Liverpool, £35m), Antef Tsoungui (Feyenoord, £250,000), Taylor Richards (QPR, undisclosed)

Burnley

Ins: Jordan Beyer (Borussia Monchengladbach, £13m), Michael Obafemi (Swansea, undisclosed), Dara O’Shea (West Brom, undisclosed)

Outs: Ashley Barnes (Norwich, free)

Chelsea

Ins: Kendry Paez (Independiente del Valle, £17m), Christopher Nkunku (RB Leipzig, £52m)

Outs: Dujon Sterling (Rangers, free), Xavier Simons (Hull, free), N’Golo Kante (Al-Ittihad, free), Kalidou Koulibaly (Al-Hilal, undisclosed)

Crystal Palace

Ins: Jefferson Lerma (Bournemouth, free)

Outs: Jack Butland (Rangers, free) James McArthur (released), Luka Milivojevic (released)

Everton

Ins:

Outs: Yerry Mina (released), Andros Townsend (released), Asmir Begovic (released), Isaac Price (Standard Liege), Niels Nkounkou (Saint-Etienne, £2m), Tom Davies (released)

Fulham

Ins:

Outs: Joe Bryan (released), Shane Duffy (Norwich, free)

Liverpool

Ins: Alexis Mac Allister (Brighton, £35m)

Outs: Roberto Firmino (released), James Milner (Brighton, free), Alex Oxlade-Chamberlain (released), Naby Keita (Werder Bremen, free), Calvin Ramsay (Preston, loan)

Luton

Ins:

Outs: Sonny Bradley (released), Henri Lansbury (released), Harry Isted (released)

Man City

Ins:

Outs:

Man Utd

Ins:

Outs: Phil Jones (released), Axel Tuanzebe (released), Ethan Galbraith (released), Di’Shon Bernard (released)

Newcastle

Ins: Yankuba Minteh (Odense Boldklub, £6m)

Outs: Ciaran Clark (released), Matty Longstaff (released), Yankuba Minteh (Feyenoord, loan), Chris Wood (Nottingham Forest, undisclosed)

Nottingham Forest

Ins: Chris Wood (Newcastle, undisclosed)

Outs: Jesse Lingard (released), Andre Ayew (released), Jack Colback (released), Lyle Taylor (released), Cafu (released), Jordan Smith (released)

Sheffield United

Ins:

Outs: Billy Sharp (released), Jack O’Connell (released), Enda Stevens (released), George Broadbent (Doncaster, undisclosed)

Tottenham