Fisher has signed a three-year contract and joins for an undisclosed fee.
The 23-year-old is the fifth signing made by North End this summer but the first since Alex Neil was appointed as manager three weeks ago.
Berkshire-born Fisher started his career in Scotland with Celtic, making his first-team debut in 2013.
Preston North End boss confident deals will get done as search for wing back and striker continue
Preston North End's predicted line-up for Championship clash at Luton Town
Preston North End boss provides injury update on Manchester United loanee and addresses specualtion linking them to another Red Devils youngster
Teenage Preston police cadet wins art competiton hosted by Larches and Savick Boxing Club to help keep troubled youths off the streets
Preston North End debutant talks making the step up, learning from Robbie Brady and Alvaro Fernandez and this only being the beginning
He spent the 2015/16 season on loan with St Johnstone and last summer moved back south of the border to join Rotherham.
Fisher made 34 starts for the Millers in the Championship last season, one which ended in the South Yorkshire outfit being relegated.
North End boss Neil said: "He's perfect in the fact that he wants to try and find his way in his career.
"He is a determined lad; he has come through Celtic's ranks, so he's used to high standards and winning games and expecting to be challenging at the top end of the table.
“He has had that experience there, which I think will certainly help him.
“I think we needed a few more bodies in defensive areas and I think Darnell will fit in perfectly for us. I think it's a great bit of business."