Williams will play a character called Noah, although the exact details are being kept under wraps, according to The Hollywood Reporter.
It is not currently clear whether the 21-year-old actor will appear in the AMC zombie drama on a guest, recurring or regular basis.
Noah is not a character featured in Robert Kirkman's Walking Dead comics.
Williams joins the likes of Seth Gilliam for season five of the hit series, while Andrew J West, Alanna Masterson and Christian Serratos are promoted to regulars.
'The Walking Dead' season five begins on October 12.