The 27-year-old joined the Gunners from Atletico Madrid on transfer deadline day after the English side paid his £45 million release clause.
Partey is set to become one of the highest earners at Arsenal, with the Guardian reporting that he is on a £250,000 per week wage.
The Ghanaian had been the target of transfer speculation all through the summer, having been linked with several elite clubs.
However, it was Arsenal who won the race for his signature after paying the midfielder’s release clause.
In a statement announcing his signing, the Premier League club said: “Thomas Partey has joined us on a long-term contract.
“The 27-year-old midfielder had been with Atletico since 2012, making 188 appearances, in which time he won both the Europa League and Super Cup in 2018, and was a Champions League runner-up in 2016.”
Partey is said to have signed a four-year contract with the Gunners, with an option to extend for another year.