The 28-year-old has spent a year at Sheffield United, but has not featured for the Blades first-team, and will be Iron manager Nigel Adkins' first signing in the January transfer window.
He started his career with Toulouse and also played for Nimes, AC Ajaccio, Kayserispor and Le Havre before agreeing a two-and-a-half year contract at Bramall Lane.
Seck could be in line to make his debut when United host Barnsley on New Year's Day.
Adkins heads into January with a host of defensive problems. Centre-backs Andy Butler and Izzy Iriekpen both have injuries, while full-backs Cliff Byrne and Marcus Williams both face suspensions.
January looks like being a busy month for Adkins, with the month-long transfer window being the only time in the season he can sign players on permanent deals.
He will also try to extend the loans of Jack Cork and Kelly Youga until the end of the season.