Best, who has made 114 appearances for Ulster, played in all four of Ireland's pool matches at the recent World Cup but has yet to feature in the 2008 Six Nations championship. He will arrive at Franklin's Gardens in the summer.
The Belfast man said: "I spoke to Jim (Malinder) and Dorian (West), who are both down to earth and spoke frankly and honestly. They are also ambitious and the facilities at Northampton are outstanding. All of that sold it for me.
"I've played against English teams and I don't have any problem with people who are bigger than me running at me. I enjoy that! The physical aspect will be different because it is so relentless but I'm looking forward to that.
"The standard of the competition is a step up from the Magners League and this is the perfect time in my career to take it to the next level.
"Myself and Roger, we started our careers together and I've played most of my games with him. I know how he plays and he knows how I play.
"But that means little when it comes to selection. Everybody's up for selection so it doesn't make any difference until the team has actually been picked," he added.
Northampton's director of rugby Jim Malinder said: "Neil is a superb addition to our squad. He has a strong physical presence on a rugby pitch and the skill to match.
"He also wants to improve as a player and as part of the team here at Northampton. All of these are attributes we look for in our players and Neil fits the part."
Commenting on the decision of a number of players to leave Ravenhill, new Ulster coach Matt Williams said: "None of the departures have come as a great shock, obviously we are disappointed to have lost these Ulstermen but they had all informed me months ago of their plans.
"In some ways, the fact that players have made their own decisions to leave makes it easier for me as some players were going to have to go anyhow. The fact they have announced their plans so early gives us a lot of time to get organised and I know they are still fully committed to the cause of Ulster for the next few months."