Dickinson, who is captaining the Titans this season, has been at Clifton Lane since 2009, having previously played for hometown club Caldy. In his two-and-a-half years he had played 63 times at both lock and in the back row, scoring four tries.
Director of rugby Jim Mallinder says that he had been following Dickinson's progress closely over the past few months and that the forward had the potential to make his mark at Franklin's Gardens.
"Time and again The Championship has shown it is a league full of quality players ready to step up into the Premiership, and Sam fits into that category," he said. "He is physically strong, very competitive and has good leadership skills. He can also play in a number of positions, which is very important as we try to successfully manage our resources within the salary cap.
"He is also very hungry to succeed as a full-time professional, and we believe that he will be a good asset to the Saints squad next season.
"We are only bringing players to Franklin's Gardens who we think can succeed at the top end of the Premiership and Heineken Cup, which is the level where we want to be playing at year in, year out.
"We've got a strong squad here at the Saints and are now in the position where we are adding to that, not making wholesale changes for the sake of it."
Dickinson says that he is excited by the prospect of playing in the black, green and gold, but also acknowledged the challenge ahead of him.
"It was a bit of a shock to get the call from Northampton," he said. "I knew that a number of clubs had been looking at me, but to get the call from a big club like the Saints at the top end of the Premiership was flattering and I had no hesitation in putting pen to paper.
"It is an exciting challenge for me. I took the step up from playing at Caldy to Rotherham, but the move to the Saints represents another massive stepping stone in my career. I'm certainly looking forward to putting in every effort in a full-time environment and am relishing the opportunity.
"I'm also looking forward to training and hopefully playing with some of the best players in the country and am under no illusions as to how difficult it will be to earn a first team shirt at Franklin's Gardens. So I'll be taking it one step at a time and trying to do my best."
Three members of the Junior Saints Academy have signed their first full-time contract to join the Senior Academy next season. Fly half Will Hooley, centre Tom Collins and prop Danny Hobbs have all impressed in either the Saints Under-19 or Wanderers teams this season, and Mallinder said that they deserve their opportunity.
"Alan Dickens, Mark Hopley and the rest of the Academy coaches are doing an excellent job bringing through talent and Will, Danny and Tom have all done really well this season," he added.
"They have trained regularly with the first team during the school holidays and have impressed us with their skill level and determination."
The trio will join the full-time squad at Franklin's Gardens once all their academic commitments have finished in the summer.