Karuru was one of many players released by Kaizer Chiefs at the end of the 2015/16 season and Usuthu came to his rescue, signing him in January after a six month spell without a club.
The 28-year-old was handed a short-term deal with the option to extend by a further year.
After starring at the COSAFA Castle Cup for Zimbabwe, Karuru has attracted interest both locally and beyond South African shores.
With AmaZulu campaigning in the top flight next season, after buying Thanda Royal Zulu's Absa Premiership status, remaining at the club appeared an attractive proposition for Karuru.
Coach Joey Antipas has confirmed that Karuru is expected back for the club's pre-season preparations.
"We have exercised the option on Karuru of retaining him and we're just waiting for him to come back from Zimbabwe after the top tournament [COSAFA Cup] that they had and he should be back any time soon," Antipas tells KickOff.com.
Karuru scored two goals in 13 league appearances for Usuthu this past season, used mostly used on the wing and also as the second striker with Mabhudi Khenyeza and Siyabonga Nomvethe preferred up front.
Usuthu will play in this weekend's KZN Premier's Cup at Umhlathuze Sport Complex in Richards Bay with the participants comprising of all of the province's professional teams - Golden Arrows, Uthongathi FC, Richards Bay FC, Real Kings, Royal Eagles and Maritzburg United.