The 30-year-old returns for a second spell at the Millers, having first joined on loan in 2013, and pens a three-year deal.
The striker was released by Hull at the end of last season, having scored five goals in 31 appearances as the Tigers survived the drop.
Eaves has similar attributes to the departed Michael Smith, who reneged on a new deal with the Millers after League One Sheffield Wednesday came in with a bumper financial package, but boss Paul Warne wanted him in his ranks alongside Smith.
Warne was a fitness coach when Eaves came to the AESSEAL New York Stadium in 2013 for a short loan spell.
He played nine times for the Millers under boss Steve Evans, scoring once.
His most successful spell was at Gillingham, again under Evans, where he scored 40 goals in 97 games across two seasons.
Eaves becomes the second signing in 24 hours following the capture of wing-back Peter Kioso.
Kioso signed for an undisclosed fee from Luton and said a chat with Warne was enough to tempt him north.
“The gaffer gave me a call whilst I was on holiday and he told me he was interested and to be honest I was sold straight away,” he said.
“The gaffer said how important the mentality was in the club and how close the group are and how he wants to do something special this year and I just want to be part of that.
“The gaffer is a big part of that. We spoke on the phone for around 20 minutes and it was a good conversation, it wasn't just about football, which I liked.”