Tokyo Metro announced that they would provide free Wi-Fi service for foreign travellers at every Tokyo Metro station except stations listed below*.
This service has started from June 1, 2016.
*Free Wi-Fi service is not available at Kita-senju Station on Hibiya Line (H21), Wakoshi Station on Yurakucho Line (Y01), Meguro Station on Namboku Line (N01), and Shibuya Station on Fukutoshin Line (F16).
TRAVEL JAPAN Wi-Fi :It may be convenient for you to download application. You may visit these sites for more details.
You can see these stickers at the station when it wi-fi is available.
Free Wi-Fi services on the subway has already available on Ginza line and Hibiya line. It also has started on Tozai line since October, 2017, on Chiyoda line since November, 2017. The free Wi-Fi service will expand to Marunouchi line by end of 2018 and other line towards 2020.
We will have more connection not only at the stations but on the trains. Internet connection sometimes fail at the moment, but I believe it will be better soon!
Have fun in Tokyo!