The 29-year-old signed for the Addicks in August 2020 scored 22 times in 71 appearances. He has also had spells at St Ives, Newport County, Peterborough United, Queens Park Rangers, Sheffield United and Scottish club Hearts.
A club statement confirmed: "Rotherham United are delighted to confirm the signing of striker Conor Washington, who has penned a two-year-deal at AESSEAL New York Stadium, subject to EFL and FA ratification.
"Washington has been snapped up by the Millers following the expiration of his contract at Charlton Athletic and becomes Paul Warne’s first signing of the 2022/23 campaign.
"He will officially become a Rotherham United player on 1st July, when his contract with the Addicks expires."
Washington has 35 caps for Northern Ireland and six goals for his country and found the net twice for the Green and White army last season in their 2022 FIFA World Cup Qualifying matches against Lithuania and Bulgaria.
The forward will join up with his new teammates for the first time when they return to training at the end of June.
Upon joining Rotherham, Washington said: “It’s great to be here, the manager and his coaching staff set their stall out very early in terms of their interest.
“I’d heard a lot about the gaffer, as well as Richie and Hammy, too. It was nice to put a bit of personality and faces to the names and the Zoom meetings were everything I expected them to be and more.
“My vision aligned with theirs. I wanted to be back at this level, competing to play week in, week out and they think I can do that.
“I still think I have a lot to learn as well and I think they can help me to push on my game further, I’m excited to get going."