Smith, 26, a revelation since joining the club from Bury in January, initially on an 18-month deal, is now contracted with the Millers until June, 2022.
He said: “It is a nice feeling to know that the club want to keep me here for an extended amount of time.
“I have really enjoyed my first six or seven months here. It is a great group of lads and to top it off with a promotion was an unbelievable end to the season. Hopefully, there are more happier times ahead.”
Proctor, also 26, who returned to the fray at the start of this season after missing virtually the whole of last term with a serious knee injury sustained in August, 2017, has signed a new contract committing his future to the club until the summer of 2021. Lancashire-born Proctor joined the Millers from Bolton in the summer of 2017 on a two-year deal.
He added: “I am absolutely buzzing. There have been more downs than I have had throughout my career over the past 18 months as well, but plenty of highs mixed in there.”
The pair have been rewarded for their form in the opening quarter of the Championship season, with Smith - a revelation since joining the club from Bury, initially on an 18-month deal for an undisclosed fee in January - penning a three-year extension.
Meanwhile, young Middlesbrough winger Lewis Wing has signed a contract extension until the summer of 2022.