It looks like a Free! - Iwatobi Swim Club movie is in the works. Entitled High ☆ Speed! - Free! Starting Days -, the film is scheduled to hit Japanese theaters on December 5, 2015.
映画「ハイ☆スピード! -Free! Starting Days-」の全国ロードショーが決定致しました！ みなさま12月5日の公開を楽しみにお待ちください！ pic.twitter.com/g3IUUDrWnJ— ハイ☆スピード！＆Free!公式 (@iwatobi_sc) March 22, 2015
Aside from the release date and the key visual above, no further details are currently known. Judging from the title, though, it's likely that the film will be a prequel.
Source: Official Free! - Iwatobi Swim Club Twitter account
Paul Chapman is the host of The Greatest Movie EVER! Podcast and GME! Anime Fun Time.