Will, who played James' on-screen nephew in the 90s American sitcom, wrote a heartfelt message on Facebook yesterday (6 January).
"Some of my greatest lessons in acting, living and being a respectable human being came through James Avery," said 45-year-old Will.
Referring to James' Fresh Prince character, Will added: "Every young man needs an Uncle Phil. Rest in Peace."
James, who played judge Philip Banks in more than 140 episodes of the Fresh Prince of Bel-Air, died at the age of 68 on New Year's Eve (31 December) after reportedly suffering complications following heart surgery.
Since James' passing, other co-stars have also paid tribute. Alfonso Ribeiro, who played his son Carlton, explained that James was like a "second father" to him.
Tatyana Ali, who played his daughter Ashley, and Will's actress wife, Jada Pinkett Smith, also paid their respects.
The Fresh Prince of Bel-Air aired from 1990 to 1996 on America's NBC network. James is survived by his wife of 26 years, Barbara Avery and his mother, Florence Avery.