The former England lock has penned a three-year deal with the club from Super Rugby side The Sharks.
Richards believes that the 33-year-old's experience at the top level will benefit the rest of his squad.
"I'm very pleased to have been able to secure Mouritz's services; he is a proven talent at the highest levels of the game and will add another dimension to our second-row options," he told the club's official website.
"He has a vast array of experience in two of rugby's best club competitions and we are looking forward to seeing him deliver the sort of performances that were his trademark during his time with Saracens.
"With three other full internationals alongside him, we have a very deep talent pool to draw on this season at lock alongside some exciting young prospects who will only benefit from having Mouritz around."
Newcastle start the new Aviva Premiership season on October 16 when they host Gloucester Rugby.