Felipe Massa said he would not have returned to Formula One for any team other than Williams.
The 35-year-old Brazilian has postponed his retirement plans to fill the space left by former teammate Valtteri Bottas, who will move to Mercedes next year. The decision came after Massa bid an emotional farewell at the final two races of 2016, but he said Williams made him an offer he could not refuse.
"My intention was always to race somewhere, as I still have the passion for racing and competing," he said. "I also have a strong love for Williams as I have enjoyed the last three years with the team, and therefore coming back to help give stability and experience to drive things forward in 2017 was something that felt right to do.
"I have a passion for racing, for competing and for fighting on the track. My return is not about seeing Formula One as the best option, but is about seeing the role at Williams as the best option. I would not have returned for any other team."
Asked if money was a factor, he said: "I won't be discussing figures or any details regarding the negotiations. All I will say is that this was the right decision and I am happy to continue as part of the team."
The sequence of events that led to Massa's return was kicked off by Nico Rosberg's decision to retire after becoming world champion last year. Soon after the shock announcement it became clear that Mercedes was targeting Bottas as Rosberg's replacement, which led to speculation that Massa would return to Williams.
"I think what happened at the end of last season with Nico's unexpected retirement sparked a unique turn of events. Valtteri has been offered a fantastic opportunity and, as a result, an opportunity arose for me.
"When the media began reporting that I might return, I was touched by the response from so many fans who wanted to see me back in the sport. That was certainly a factor in the decision, so I'd like to thank the fans for their support. But, at the end of the day, when I received the call it was an offer I couldn't refuse. It was Williams!"
Massa is on a one-year deal and insists he has not given any thought to what comes in 2018.
"At present this is a deal for 2017 only and I have not thought beyond."