In August 2014, Charlie Hunnam, Ritchie’s choice for the role, was cast to play King Arthur. Elizabeth Olsen was in talks for the female lead, but on 18 September, Àstrid Bergès-Frisbey landed the role instead. On 14 November, Jude Law was in talks to play the lead villain role in the film. On 11 February 2015, Eric Bana was added to the cast to play Uther, the father of King Arthur. Mikael Persbrandt joined the film on 6 March 2015, to play a villainous role. Although there were reports Idris Elba had been cast, he would confirm, in a Reddit AMA, this was a rumour.

In April 2014, Warner Bros. set the film for a 22 July 2016 release, alongside Lights Out, but it was then moved by Warner Bros. to 17 February 2017, alongside Maze Runner: The Death Cure. Warner Bros. moved the release date to 24 March 2017. The title was changed to King Arthur: Legend of the Sword in July 2016. In December 2016 the release date was again moved, this time to 12 May 2017, possibly so as not to compete with CHiPs.

Filming in Windsor Great Park was underway in February 2015, then later in North Wales from 2 March 2015. Later on 10 March 2015, Ritchie tweeted a photo and confirmed the first day of shooting. In April 2015, filming took place in Snowdonia, where locations used were Tryfan, Nant Gwynant near Beddgelert and Capel Curig. Early in July filming continued in the Shieldaig, Loch Torridon and Applecross areas of Wester Ross in the Scottish Highlands. One day of filming also took place at The Quiraing on the Isle of Skye. Filming also took place at Warner Bros. Studios, Leavesden.

The trailer was released on 23 July 2016, during the 2016 San Diego Comic-Con.

It was originally planned for an IMAX release on 22 July 2016, as evident in the Comic-Con trailer, but was cancelled due to it being postponed and only received non-IMAX presentations. The first extended TV spot was released on 22 January 2017, and was shown on the 2017 NFC Championship Game on Fox Broadcasting Company between the Atlanta Falcons and the Green Bay Packers and the 2017 AFC Championship Game on CBS between the Pittsburgh Steelers and the New England Patriots.

A second trailer was released on 20 February 2017. The third and final trailer was released on 1 April 2017. All in all, Warner Bros. spent $135 million on marketing the film.

The film was shown at select AMC Theatres in a special preview on 27 April 2017, in a promotional event titled “King for a Day.” Demand for tickets led AMC to expand the event to 200 theatres.

King Arthur: Legend of the Sword was released on DVD, Blu-ray, 3D Blu-ray and Ultra HD on 8 August 2017.