In July 2000, Essien was signed by Division 1 club Bastia and made his debut for the club on 30 September 2000 in a league match against Metz, appearing as a substitute for captain Laurent Casanova. Essien did not immediately command a consistent place in the team, being deployed in various positions in the back four. He ultimately finished the 2000–01 campaign with only 13 league appearances and one goal, which came in the return leg against Metz in a 3–2 defeat.
The following season, Essien was given an opportunity by manager Robert Nouzaret to play in central midfield and flourished in this new role. Essien formed midfield partnerships with Nicolas Dieuze and Cyril Jeunechamp and scored vital goals in victories over Nantes, Lorient and Guingamp. On 12 January 2002, Essien scored the equalising goal against Marseille at the Stade Vélodrome to even the match at 2–2. He also scored a goal in the Coupe de la Ligue against Le Mans. Future Chelsea teammate Didier Drogba converted a goal for Le Mans in the match.
In the 2002–03 season, Essien developed into an undisputed starter under new manager Gérard Gili. However, his tenacious and physical style of play also began to take form as he collected a staggering 12 yellow cards in league play. He opened the new campaign by scoring on his season debut against Lens in a 1–1 draw and converted five more league goals, which included one in a surprising 2–0 victory over the defending champions Lyon. Following the campaign, in which Bastia finished mid-table for the third consecutive season, Essien drew interest from Paris Saint-Germain, Lyon and Marseille.