Organizers announced their coup on Monday, claiming Federer had agreed to play in the tournament, which switched from clay to grass for the first time back in June, beyond next year.
Having been post-Wimbledon this year, the ATP Stuttgart Open will be played before the grass court Grand Slam in 2016.
"I watched the first (grass court) edition on television, it had a great atmosphere," Federer said.
"I've always wanted to play Stuttgart, but it was always the wrong time after Wimbledon," he said.
"Now that they've moved dates, the chance to play the MercedesCup is very exciting — I can't wait."
The seven-time Wimbledon champion also said he was happy there would be an extra week between the end of the French Open and the start of Wimbledon next year, giving players more time to adapt to the switch from clay to grass.
"It's genius to have an extra week. It doesn't sound like much, but for us players it's a lot," said the 34-year-old.
"I love playing on grass — always have, always will. To play another tournament on grass is great for me. I can manage my energy and schedule. It's all about winning titles."
Previously, Federer had been coy about his long-term future, committing only to his schedule next year.
"Wimbledon is a big one. Rio as well but it's not my number one priority. It starts with the Australian Open which is just around the corner," he said following his defeat to Novak Djokovic in the title match at the ATP Tour Finals on Sunday.
"Once that's over then I look more towards the future."