| MJ Mentz | |
| Full Name: | Marthinus Johannes Mentz |
| Birth Date: | 21 July 1982 |
| Birth Place: | Ermelo, South Africa |
| School: | Hoërskool Ermelo |
| University: | North-West University, Potchefstroom |
| Position: | Winger / Fullback / Fly-half |
| Youthyears1: | 2002 |
| Years1: | 2002–2004 |
| Apps1: | 27 |
| Points1: | 45 |
| Years2: | 2005–2008 |
| Apps2: | 66 |
| Points2: | 140 |
| Years3: | 2009–2012 |
| Apps3: | 38 |
| Points3: | 40 |
| Repyears1: | 1998 |
| Repteam1: | South Africa U16 |
| Repyears2: | 2000 |
| Repyears3: | 2003 |
| Repteam3: | South Africa U21 |
| Repsevensyears1: | 2007–2010 |
| Repsevensteam1: | South Africa Sevens |
| Repsevenscomp1: | 18 |
| Coachyears1: | 2013–2014 |
| Coachteams1: | (assistant) |
| Coachyears2: | 2015–2016 |
| Coachteams2: | (head coach) |
| Coachyears3: | 2017–present |
| Coachteams3: | (backline coach) |
| Clubupdate: | 28 October 2015 |
| Repupdate: | 28 October 2015 |
Marthinus Johannes "M.J." Mentz (born 21 July 1982, Ermelo, South Africa) is a former South African rugby union player and currently the backline coach of Currie Cup side the . He regularly played as a winger or a fullback.
He played for the, and in domestic South African rugby during his playing career which spanned 2002 to 2012. He also represented South Africa at Under-16, Under-18 and Under-21 level and represented the South African Sevens in 18 tournaments between 2006 and 2011, including the 2010 Commonwealth Games in India where his team won the bronze medal.
He wrapped up his playing career at the Pumas, retiring after the 2012 season. The Pumas appointed him as a coach and assistant to head coach Jimmy Stonehouse. When Stonehouse left to join Japanese Top League side Toshiba Brave Lupus at the start of 2015, Mentz was named as his successor.[1] He guided the Pumas to their first ever Vodacom Cup title in 2015, beating 24–7 in the final, and to sixth position in the 2015 Currie Cup Premier Division.
On 2 November 2015, the Pumas announced that Mentz was reappointed as head coach until the end of 2017.[2]