Seabrook, 25, is in the last year of a three-year deal and was to be a restricted free agent at season's end. There are details to be worked out, according to the source, but the extension would be for five years and worth between $5.6 million and $6 million a year.
"We're getting close [to a deal]. It's certainly moved along faster in the last week or so," Blackhawks GM Stan Bowman said Saturday. "Nothing yet. I expect something soon hopefully.
"He's been a great player and I think he's going to get even better."
Seabrook last season helped the Blackhawks win their first Stanley Cup title in 49 years. He also was on the Canadian team that won the gold medal at the 2010 Vancouver Olympics.
Seabrook has four goals and 29 assists and a plus-2 rating this season for the Blackhawks, the only team he's played with in six NHL seasons.
During an interview Feb. 10 on ESPN 1000, Blackhawks general manager Stan Bowman said the team was "aggressively trying" to sign Seabrook to a new contract.
Jesse Rogers covers the Blackhawks for ESPNChicago.com and ESPN 1000. ESPN.com's Pierre LeBrun contributed to this report.