The group, called Volume 12, announced on its Facebook page it had set the mark late in the first quarter. The decibel reading taken during a sack of San Francisco quarterback Colin Kaepernick in the first quarter following a one hour weather delay was 131.9 decibels.
The crowd later bested the mark with a reading of 136.6 decibels after a third quarter interception.
The previous record for "loudest crowd roar at a sports stadium," was 131.76 decibels, set in 2011 in Turkey at the Ali Sami Yen Sport Complex Turk Telekom Arena during a soccer match between Galatasaray SC and Fenerbahce.
The group footed the bill for the Guinness World Records official to come to Seattle and wasn't affiliated with the team.