Doctors had no choice but to sever the limb “to keep him alive”, relatives say in a statement on his Instagram page.
“This development has been hard on us all but we’ve had to accept it as Daddy’s new reality,” it reads.
Okafor, 62, has starred in dozens of Nollywood films including Keziah, 9 Wives and Mr Ibu in London.
His family have not disclosed the cause of the illness that led to his leg being amputated.
The statement adds that the actor has undergone seven rounds of surgery, and needs one more.
Nigerian media report that some of Okafor’s medical expenses have been paid by former Senate President Bukola Saraki.
His family is appealing for fans and well-wishers to pay the remainder, without specifying the amount.
Such appeals from Nollywood actors are not uncommon, says the BBC’s Annette Arotiba in Nigeria.
This has raised questions from many, asking why actors who appear to be well-off cannot afford to pay for their own medical bills.
Most Nigerians rely on private medical care which can be costly.
Industry insiders say pay rates in Nollywood vary, with top actors receiving up to 3m naira ($3,700; £3,000) and the smallest parts being unpaid.