Floyd's game-winning score on a throw by Carson Palmer enabled the Cardinals (10-5) to keep their playoff hopes alive and handed the Seahawks their first home loss since Dec. 24, 2011.
Palmer overcame four interceptions, completing 13-of-25 passes for 178 yards.
The Cardinals held the Seahawks to 192 total yards total offense.
Russell Wilson managed 108 yards with a TD and an interception on 11-of-27 passing for Seattle (12-3), which was seeking to clinch the NFC West and home-field advantage throughout the playoffs.
The Seahawks could still win the division if San Francisco loses or ties against Atlanta Monday.