Arians recently left Pittsburgh, where he held the same position. He will join the staff of new coach Carl Pagano, the person said on the condition of anonymity because the team has not yet announced the hiring.
Pagano was defensive coordinator in Baltimore and faced Arians' offense twice a season.
The Steelers said 10 days ago that the 57-year-old Arians was retiring, but he resurfaced with the Colts, who are rebuilding. With Peyton Manning's health uncertain following three neck surgeries, including one that cost him the entire 2011 season, Arians might be coaching Stanford star Andrew Luck next season. Indianapolis (2-14) has the first overall selection in April's draft.
Arians was Manning's first offensive coordinator in the NFL, in 1998.