The Italian rode for Yamaha from 2004-10 before switching to Ducati, where he struggled to continue his run of success and could only manage three podium finishes across two seasons.
He will resume his relationship with 2008-10 team-mate Jorge Lorenzo, the current MotoGP champion, at Yamaha next year.
"It's a return for me, and the feeling was like having never been away," Rossi told Gazzetta dello Sport after testing with Yamaha at Valencia and Aragon.
"After seven years spent here there are many people I know already."
Yamaha had abandoned the official MotoGP test at Valencia after Tuesday due to poor weather. It travelled to Aragon instead, only for rain to hit that test as well, while conditions ultimately cleared up in time to allow some dry running at Valencia.
"We've moved from Valencia to Aragon because we were hoping for good weather. Instead we have found more rain," said Rossi.
"And to add insult to injury - because we needed to test two or three important things to work on next year's bike - good weather returned to Valencia and they were able to run there.
"We needed to get some mileage in the dry, instead we'll have to wait for next year before getting to the track again."
Although Rossi was limited in how much he could comment on the Yamaha while still officially contracted to Ducati, he hinted that there had been plenty of familiar feelings.
"It's different from the one I had ridden, it has a more powerful 1000cc engine, but as for the rest it doesn't seem so different from back then," he said.