Antoine Fuqua had no money to pay James Horner to compose the film, due to the film's short budget. However, Horner did not care as he loved the idea of the film, and he decided to do it for free.

Eminem was originally scouted for the role, which he accepted and started filming. The film had to be put on hold as Eminem wanted to focus on his music career, so the role was then given to Jake Gyllenhaal.

This is the next to last film scored by composer James Horner, before dying in a plane crash on June 22, 2015. His last film in release, The 33 (2015), was released later in the year, his final score for the western remake The Magnificent Seven (2016), also directed by Antoine Fuqua, is set to be released in September 2016.

Some prints of the film, at the end of the credits, have "In Memory of Our Friend James Horner," who died in a plane crash days before the film opened for release.

The film was originally considered as an unofficial follow-up to the movie 8 Mile (2002), with Eminem reprising his role. The script eventually evolved and that idea was scrapped.

"Southpaw" is the term given to unorthodox stance (left-handed) boxers. However, Jake Gyllenhaal is right-handed, therefore orthodox. He only adopts the southpaw stance in the final fight under instruction from his trainer, Tick. Eminem was originally cast as Billy Hope and is left-handed, making the film title more suitable.

Writer Kurt Sutter's first film script.

The song that plays at the end of this film, "Wise Man" by Frank Ocean, was originally written to be in Django Unchained (2012). Quentin Tarantino wasn't able to find a scene to place the song in, so it was eventually scrapped from the final cut.

The boxing scenes shot in Las Vegas and at Madison Square Garden were all done at the same location over the course of two weeks. The Indiana University of Pennsylvania (IUP) was used to simulate both fight locations.

Since Jake Gyllenhaal had to lose thirty pounds for his role in Nightcrawler, he only needed to gain fifteen pounds for the role of Billy Hope by training twice a day, seven days a week.

Jake Gyllenhaal was inspired by Miguel Cotto, due to his impressive career and evolution as a boxer.

The scene where Billy Hope meets Tick Willis at his boxing school was actually filmed in Pittsburgh, but it was made to look like the Bronx section of New York City. The street signs were changed, New York cabs were brought in, and fake graffiti was painted on the walls, all to give the neighborhood an authentic N.Y. neighborhood look.

Eminem appeared in two songs for the movie, "Phenomenal" and "Kings Never Die," but he was Executive Producer for the rest of its soundtrack.

Skylan Brooks's character, Hoppy, pays homage to Eminem's character in 8 Mile (2002), named B-Rabbit.

Lupita Nyong'o was cast as Angela Rivera but dropped out, due to scheduling conflicts with Star Wars: Episode VII - The Force Awakens (2015).

Ryan Gosling, Bradley Cooper and Jeremy Renner were all considered for the role of Billy Hope.

The title of the film is a reference to the eponymous stance traditionally adopted by a left-handed boxer. The term "southpaw" is also slang for a left-handed person.

Originally, Harry Gregson-Williams was announced as the film's composer. Gregson-Williams even stated in an interview in late 2014 that he was already writing the score. Not too long after that, James Horner was announced as the composer.

The writing credits in early promotional material read, "Story by Kurt Sutter. Screenplay by Sutter and Richard Wenk." Sutter received sole "Written by" credit following an arbitration conducted by the Writers Guild of America.

Eminem worked on the movie's soundtrack but did not appear in the movie itself.

Forest Whitaker doesn't appear on screen until 52 minutes into the movie.

"Southpaw" is actually a baseball nickname for any lefthanded pitcher, since home plate is always west of the pitcher's mound.

Jeff Wincott auditioned for the film.

Released the same year also based on boxing, Creed (2015), both Billy Hope and Adonis Creed who are stars on the up rise.

Although the storyline depicts a particular sequence of a boxer suffering significant losses both literaly and literarily, Billy Hope's character resembles that of former Canadian Heavyweight Champion George Chuvalo and his post-boxing life struggles toward coping with several familial tragedies and turmoil.

Jake Gyllenhaal, Rachel McAdams, and Forest Whitaker who star in the film star in the MCU. Gyllenhaal, who played Billy in the film played Mysterio in Spider-Man: Far from Home (2019). McAdams, who played Maureen in the film played Christine Palmer in Doctor Strange (2016). Whitaker, who was Titus in the film played Zuri in Black Panther (2018).

Rachel McAdams said filming her death scene was a long process that took around 7 hours. She was covered in blood and her co-star Jake Gyllenhaal was very supportive.

To prepare for her death scene, Rachel McAdams watched death videos on YouTube.

It was stated that: "Maureen's date of birth, written on her tombstone, is October 2, 1980. That means she gave birth to her daughter at her 24th birthday, October 2 2004 (II-X-MMIV Leila tattoed on Billy's chest)" However, II-X-MMIV (Layla's birthday) could also be read as February 10, 2004. This is the American way of expressing dates, MM-DD-YYYY. The other format DD-MM-YYYY is usually used in every other country.