The defender, 29, has returned to the club four years after leaving for Reading and having been released by Crystal Palace earlier this summer.
He came through Watford's academy, establishing himself at the club in the 2006-07 season before their relegation from the Premier League.
After 247 appearances Mariappa joined Reading, who were similarly relegated back to the Championship, before signing for Palace a year later.
The last of his appearances there came in May's FA Cup final defeat to Manchester United.
The Jamaica international's return to Vicarage Road was announced on the Hornet's official website.
