Most of the characters (Seth Bullock, Al Swearengen, Sol Star, Reverend Smith, the Metz family, et cetera, in addition to the more famous Wild Bill Hickock, Calamity Jane, and Jack McCall), have real-life counterparts.

A feature film based on the series has been rumored for many years, but the cast has cited the difficulties of coordinating the schedules of everybody involved as a main obstacle. In 2016, HBO head Mike Lombardo said David Milch had pitched a general script, and that the network is fully committed to the movie.

The word "fuck" and its derivatives are used 2,980 times throughout the series.

The series takes place from 1876 to 1877.

Garret Dillahunt pursued the role of Seth Bullock, but Timothy Olyphant was already cast. The only role that was available at the time, was of Doc Cochran, so Dillahunt auditioned for that. He played the recurring role of Jack McCall in season one. Dillahunt was then considered for the role of George Hearst in season two, but it was decided that Hearst would not appear on-screen until the season finale. Dillahunt played the recurring role of Hearst's employee, Francis Wolcott.

Powers Boothe was originally cast as Al Swearengen. But Boothe fell ill before the pilot was to start filming. Boothe was replaced by Ian McShane, and then was given the supporting role of Cy Tolliver.

George Hearst was the father of William Randolph Hearst, the famous newspaperman on whom Citizen Kane (Citizen Kane (1941)) was based, and the great-grandfather of Patricia Hearst. When Hearst tells Merrick that he will start his own newspaper in Deadwood to tell lies for his side, it is a reference to the fact that W. R. Hearst is largely credited with the creation of the concept of "yellow journalism", and the use of his own newspapers to shape, and even create political and social opinion and actual events. The most famous example of this was what many historians characterize as W. R. Hearst's whole cloth creation of the Spanish-American War through his newspapers' inflammatory and lucrative headlines.

For a long time, there was a bar called "Bullock's Tavern" in Amherstburg, Ontario, which was originally owned by Seth Bullock's parents, and was also the place he was born.

Until the end of season two, the character of Ellsworth did not have a first name. When it was decided to give him a first name, the actor portraying the role, Jim Beaver, requested that he be given the first name Whitney, after Whitney Ellsworth, Producer of Adventures of Superman (1952), whom Beaver knew from his research for a book on the life of "Superman" star George Reeves.

The role of Al Swearengen was originally written for Ed O'Neill. O'Neill screentested for the role, but HBO executives did not want to cast him, because of his fame as Al Bundy from Married with Children (1986).

Wild Bill Hickok was portrayed as an older man in the series, when in fact he was only thirty-nine years of age when he died.

Fifteen members of the cast of this show, also appeared in Sons of Anarchy (2008). The list includes Tony Swift (Prospector/Biker), Tim DeZarn (Townsman/Nate Meineke), Kevin P. Kearns (Pasco/Luke), Dan Hildebrand (Shaughnessy/Tim Driscoll/Sean Casey) , Julie Ariola (Countess/Mary Winston ), Cleo King (Aunt Lou Marchbanks/Neeta), Dayton Callie (Charlie Utter/Chief Wayne Unser), Paula Malcomson (Trixie/Maureen Ashby), Robin Weigert (Calamity Jane/Ally Lowen), Titus Welliver (Silas Adams/Jimmy O'Phelan), Jamie McShane (Ned Mason/Cameron Hayes), Ray McKinnon (Reverend H.W. Smith/Lincoln Potter), Jim Cody Williams (Terrence/Uncle Vinky), Kim Dickens (Joanie Stubbs/Colette Jane), and Keone Young (Mr. Wu/Bohai Lin).

HBO offered the chance to David Milch to wrap the series in a shorter fourth season, but he declined to do it on those conditions. However, when Chris Albrecht was asked about it, he said that they also told Milch that HBO would give him a full twelve-episode season, if it was what he needed to wrap the show. Milch told them he would think about it over a weekend, but the news about the show possibly being cancelled reached the press to such a speed, that that conversation never happened, and Milch just moved on to develop John from Cincinnati (2007).

The show's cancellation was a shock to the cast. During the third season, Timothy Olyphant and Ian McShane successfully renegotiated their contracts for higher pay, and HBO retroactively paid them the increased rate for the nine episodes of the season already shot. The show was cancelled a few weeks later.

Despite appearing in thirty-two out of thirty-six episodes, Jeffrey Jones (Merrick) was listed as a guest star in the first season. Similarly, Gerald McRaney (George Hurst) received a "special guest star" credit in season three, even though he appeared in all twelve episodes of that season.

At one point, David Fincher was set to direct the pilot.

According to a 2004 Jewish Journal of Greater Los Angeles interview with show creator David Milch, when John Hawkes first met Milch to audition for the role of Sol Star, Hawkes told Milch that he was not actually Jewish (unlike both the real-life Star and the "Deadwood" character). Milch's response was to ask Hawkes, "Have you ever felt shame or sadness or ostracized?" When Hawkes responded, "Every day". Milch told him, "Then you're Jewish."

The word "cock sucker" is used a total of 273 times.

The "nude" portrait of a woman on a couch by the door in Al Swearengen's office, is the same used in Boardwalk Empire (2010). Micky Doyle references it to Van Alden in the scene at his desk.

Certain actors and actresses that were eventually cast in the series, initially auditioned for different roles. For example, Paula Malcomson (Trixie) initially auditioned to play Alma Garrett and W. Earl Brown (Dan Dority) pursued the role of Jack McCall.

Two actors in this series have daughters who played minor roles (as prostitutes). Parisse Boothe, the daughter of Powers Boothe, played Tess in five episodes, and Fiona Dourif, the daughter of Brad Dourif, appeared in three episodes as the "Chez Ami Whore".

The real Seth Bullock was born in Amherstburg, Ontario.

David Milch had originally planned to write a series set in Rome. However, when he found out about the series Rome (2005) that was then in development at HBO, he decided to write a Western.

Doc Cochran's first name is Amos.

Ellsworth is also the name of the Air Force Base located near Deadwood, South Dakota.

Alice in Chains guitarist/vocalist, Jerry Cantrell, was a big fan of the show and even appeared in the background of one scene from season one next to Pantera's bassist Rex Brown. Cantrell is also good friends with actor W. Earl Brown, who made an appearance on the Alice in Chains mockumentary Alice in Chains Twenty-Three (2013).

Alice in Chains guitarist Jerry Cantrell gave W. Earl Brown the idea on how to end the fight between Dan and Captain Turner.

While Wild Bill Hickok was 39 years old when he died, Keith Carradine, who plays him, was 55 years old at the time this series was produced.

Someone else has already noted that the insulting term "cocksucker" is said 273 times in the series. It is further notable that this term is not clearly in use in the English language before the 1890s; the series occurs beginning in 1876.

The series shows Seth Bullock and Sol Star witness Wild Bill Hickok's arrival in Deadwood. However, in reality, Wild Bill arrived in Deadwood two weeks prior to Bullock. Bullock arrived in Deadwood on August 1, 1876, the day before Bill was killed by Jack McCall.

In season two, the actor playing Francis Wolcott, Garret Dillahunt, also played Jack McCall, the killer of Wild Bill Hickock, in season one. David Milch has a habit of reusing actors.

The corset that Alma Garret (Molly Parker) takes off before sleeping with Seth Bullock (Timothy Olyphant) for the first time, is called the "Merry Widow". Appropriate, considering her husband's recent death.

In real-life, Al Swearengen was only thirty-one years old when Wild Bill Hickok was shot in Deadwood, At sixty-two, Ian McShane was older than the historical Swearengen's entire lifetime of fifty-nine years.