The 26-year-old had been linked with a move to PSV, but has opted to return to the Pro League, three years after moving to Portugal from Standard Liege.
After 111 appearances for Porto in all competitions, Defour was deemed surplus to requirements and becomes the fifth player to depart ahead of the new season.
In an announcement to the Portuguese Securities Market Commission (CNVM), Porto confirmed the deal with Anderlecht, with the fee potentially rising to €6.5m.
It read: "Futebol Clube do Porto, under Article 248.1 of the Securities Code Securities, hereby informs the market that it has reached an agreement with Royal Sporting Club Anderlecht for the transfer, permanently, the registration rights of the player sports Steven Defour professional football for the amount of € 6,000,000.
"Please note that this agreement provides for the payment of variable remuneration at the overall amount receivable could reach € 6.5 million."
Defour represented Belgium during the recent World Cup in Brazil, but saw his tournament end early after a red card against South Korea.
Anderlecht are yet to confirm the deal and present Defour as their new signing.
