The win made it the first time Righetti has won their league since 1984, when it was the Northern League.
Both teams played hard but Righetti's defense came up big by forcing a fumble to seal the win.
Righetti High QB T.J. Jordan said: "It feels great to win league, because it hasn't been done since 1984".
Arroyo Grande sophomore phenom Seth Jacobs left the game in the second quarter with a back injury.
Righetti High finishes the season 8-2 and will host Newbury Park in the first round of CIF playoffs.