Massa was already expected to stay, as he was under contract, but it was confirmed he will be partnered by Bottas, after the team said it had taken up an option.
Team principal Frank Williams said: "Felipe is a pleasure to work with, and combines raw pace with a wealth of experience. As for Valtteri, we have known for many years he is a very special talent."
Massa joined Williams following his release from Ferrari last year and he secured the team's first pole since 2012 at this year's Austrian Grand Prix.