Pioli left AC Milan at the end of last season after leading the club to second in Serie A but failing to challenge rivals Inter, finishing 19 points behind.
He has been out of work since but will now return to the touchline with Al Nassr, who are seventh in the Pro League after three games having won one and drawn two.
They also drew to Shorta in the AFC Champions League and that cost Castro his job, with Pioli quickly being secured as his successor and the boss of a number of top players including Cristiano Ronaldo and Sadio Mane.
Pioli has been a senior coach since 2003 and has spent his entire career in Italy until now, where he has coached Inter Milan, Fiorentina and AC Milan among others.
It was at Milan that he won the only trophy of his career thus far with him leading the club to the Serie A title in 2021/22, becoming a Rossoneri legend in the process.