The 35-year-old Steyn, who went unsold in the 2018 and 2019 auctions of the Twenty20 tournament, last played for the now defunct franchise Gujarat Lions in 2016.
He had played 28 IPL matches for Bangalore between 2008 and 2010, claiming 27 wickets, and is set bolster the flagging fortunes of the Virat Kohli-led side which has lost all their six games so far.
Steyn is expected to be available for selection from Bangalore's ninth league match against Kolkata Knight Riders on April 19.
Coulter-Nile, who is suffering from a stiff back, had not joined the Bangalore team.
Bangalore, who have former Proteas coach and batsman Gary Kirsten as their head coach, are currently placed bottom of the eight-team table.
They will take on Kings XI Punjab on Saturday (16:30 SA time).
There are now 11 South Africans in the IPL:
Sunrisers Hyderabad: -
Chennai Super Kings: Faf du Plessis, Imran Tahir
Rajasthan Royals: -
Royal Challengers Bangalore: AB de Villiers, Heinrich Klaasen, Dale Steyn
Mumbai Indians: Quinton de Kock
Kolkata Knight Riders: -
Kings XI Punjab: David Miller, Hardus Viljoen
Delhi Capitals: Chris Morris, Kagiso Rabada, Colin Ingram