The 26-year-old was offered a new contract at Hillsborough, but rejected it as it was only a 12-month deal and he joins fellow ex-Owls Joe Mattock and Lewis Buxton in crossing the border.
Maguire, who spent three years with Wednesday, is a winger who can also play as a striker and he becomes the 11th summer signing for the Millers.
Boss Steve Evans, who has been chasing Maguire's signature all summer, is delighted to get his man.
“This young man is a player I have admired since his days at Aberdeen and he was a lad that was on my targeted list of players at the end of last season," he said.
"We have had to be patient as we gave this young man the time to speak to a number of Championship clubs who wanted him to join them.
“We love a player who can play in more than one position, and we all know that Chris Maguire can play as a striker or indeed a wide position, he has great qualities in both departments.
“He joins what is already a talented squad and in the next week or so I would envisage that squad maybe strengthened further.
“This has been a summer transfer window that has seen us with a very specific targeted approach for players. The focus, attention to detail, player references and what they will add to the Millers has been at the forefront of our thoughts in everything that we have done.
“Once again I must thank my Chairman Tony Stewart for his continued and everlasting support.”
