The official website of the anime series Free! announced a new TV anime series premiering Summer 2018. A teaser message was seen at the end of the Free! Take Your Mark anime movie which premiered in Japan on October 28.
— 「Free!」シリーズ公式 (@iwatobi_sc) October 30, 2017
Previews for the upcoming TV anime series have begun advertising in Japan. The official website stated additional details will be announced soon.
— たま(→) to 2020 Ø執行済 (@melissa1974098) October 30, 2017
— ??アララギ?☀️4/28タ39b (@0miiim02) October 30, 2017
Edited to include new visual and official Tweet.