The 27-year-old makes his return to the reigning Malaysia Super League champions after spending a season in the Mexico Liga MX in the 2017 season.
It wasn't a hugely successful season for the Argentine there in Mexico's top division, making only nine appearances while contributing one a solitary goal and an assist each.
Compared to his 2016 season with JDT where he banged in 30 goals in the entire season, with 17 of those coming in the 22 Super League matches.
"I am very happy to return to play for JDT. It is a club that I love very much and I feel very comfortable with. Next season will be hard, as I am confident that all teams will be prepared with better tactics and playing styles."
"However, it is the same at JDT and we as a team are working very hard to ensure we win all competitions," Diaz was quoted on the Johor Southern Tigers' Facebook page.
As a forward, goals remains the big target for Diaz as he aims to replicate his output from when he last put on the JDT shirt. The familiar partnership with fellow returnee in Luciano Figueroa is also another plus point for Diaz.
A player who has a knack of finding himself in the right places inside the penalty box despite not being an out-and-out striker, Diaz is also a big threat cutting in from wide positions as he possess good dribbling abilities.
Having left the club when it was still under the management of Mario Gomez, Diaz will no doubt have to work hard to adapt to the different methods of Ulisses Morais. The head coach from Portugal going into his first full season with JDT having led the team to success in retaining the Super League title last season.
"Personally I would like to finish as the top goalscorer again just like in 2016, but the important thing is that JDT can win all matches. Luciano Figueroa and I know each other very well, having played together in 2015. We will surely work hard to show good performances to give the best for the team."
"As for head coach Ulisses Morais, I have just got to know Morais and think he is someone with a positive attitude and always willing to guide when it comes to tactics. I'm optimistic that JDT will be stronger next season and we will work hard to defend the Super League title," added Diaz.
Diaz first immediate task is to find his form back in time for when JDT go into the crucial preliminary round AFC Champions League playoff against Thailand's Muang Thong United in late January.