Sullivan had been scheduled to make $9.5 million over the next two seasons, or $4.75 million per year.
Under the terms of the extension, Sullivan will make $7 million in 2015, $5.5 million in 2016 and another $5.5 million in 2017, according to the source.
Sullivan will get a $1 million signing bonus as part of the new deal, according to the source. That bonus, in addition to the $1 million of signing bonus proration still left from Sullivan's old contract, brings the center's salary cap hit to $7.33 million in 2015 and increases his cap number to a total of $5.83 million in 2016 and 2017.
"I want to start by saying thank you to the Minnesota Vikings for putting their faith in me as a person and football player," Sullivan said in a statement released by the team. "The Vikings have been with me through professional highs and lows, and have stood by my side and supported me through personal hardships. My ability to express the true depth of my gratitude comes from my actions on and off the field.
"I intend on doing absolutely everything I can to ensure we attain our ultimate goal of a Super Bowl championship on the field, and that I live my life in a way that is commensurate with that championship standard off the field. One of the most exciting aspects of signing this extension is the trajectory of the organization. We have a talented roster, a state of the art facility on the horizon and the best fan base in the NFL. After a year with coach [Mike] Zimmer at the helm, I couldn't be more excited about our team's future. Now, it falls on us as players to fulfill our potential and to execute and I intend on making sure that happens."
Sullivan has been a reliable player in Minnesota, starting 93 of the Vikings' past 96 games. A sixth-round draft pick in 2008, the 29-year-old Sullivan has spent his entire seven-year career with the Vikings.