He arrives following a successful trial spell and returns to the Iron after originally being here in the 2019-20 season, during which he played 21 times and scored the winner in a 2-1 victory over Port Vale in November 2019.
After that spell, he spent two seasons at Barrow in League Two, where he appeared 33 times.
The 28-year-old originally arrived at the club following two years at Rochdale where he made a total of 52 appearances, scoring three goals.
After coming into the United Kingdom in 2005 from South Africa, he was spotted playing for local side Southside by Peterborough United in 2010, subsequently signing as a scholar before being awarded a professional contract in 2011.
In five years at London Road, he scored score five goals in 89 appearances before signing for Stevenage in August 2016.
While with the Boro, he made 24 appearances, including 20 league starts in his solitary season at the club before being tempted by a move to Spotland in July 2017, under the management of current Iron boss Keith Hill.
Hill told Iron Player: "He's somebody I trust and who I'm going to put my belief in. He's at a great age for a footballer with respect to experience. He has to crunch the games out, we know that, and I'm not stood here trying to create a magic trick. I know Kgosi really well, he knows me and the demands I'm going to put him under. I'm expecting him to crunch the games and the training days out as that's where it's really important.
"He's versatile too. He's got to get his robustness back and if we get him on the training pitch and playing games, he's a fabulous player. I don't believe the Scunthorpe public has seen the best of him yet."
Ntlhe represented his country five times at Under-20 level, playing in the Cape Town International Challenge against Argentina, Nigeria, Ghana and Brazil in 2012.
He was subsequently called up to the South African national squad for an international friendly against Brazil in June 2013, before making his international debut as a 67th minute substitute in a 1-1 draw against Morocco in the following October.
The transfer is subject to league and FA ratification.
