Square Enix has announced that Final Fantasy XIII has arrived on both Steam and for download directly from SE. FFXIII's other two episodes will be released prior to March 2015.
FINAL FANTASY XIII expands on the rich traditions of the series, with Producer Yoshinori Kitase, Director Motomu Toriyama, Character Designer Tetsuya Nomura and Art Director Isamu Kamikokuryo combining their creativity to deliver a beautiful FINAL FANTASY universe. Additionally, Composer Masashi Hamauzu presents a sweeping soundtrack while Movie Director Takeshi Nozue and his team of artists adorn the game with seamless cinematic sequences. The FINAL FANTASY XIII trilogy began in March 2010 with the release of the original FINAL FANTASY XIII and continued with its sequel, FINAL FANTASY XIII-2, released in February 2012. The most recent release in the FINAL FANTASY XIII series, LIGHTNING RETURNS™: FINAL FANTASY XIII, launched on February 11, 2014. The series has been widely successful, shipping over 11 million units worldwide.
Check out the Final Fantasy XIII Steam page.