The 28-year-old, who joined the club from Saracens in 2009, has committed his future for the next three years, according to BBC Sport.
"I have enjoyed my time at the club and, whilst we are currently at the foot of the Premiership, there is a good strong team spirit," Seymour told reporters.
"We feel we can rescue our season and reward our loyal fans. I look forward to spending my future years at the club."
Sale currently sit bottom of the Premiership table.