The former Springboks Under-20 captain, who has made ten appearances for Bulls this season, will provide cover for the duration of the Rugby World Cup and will join the Capital club subject to a medical.
Steenkamp can operate as a flanker or at No. 8 and played a key role this season as Bulls reached the Super Rugby knockout stages for the first time in six years.
He also captained the Blue Bulls' Under-19 side in the Provincial Championship season in 2012 and led the Junior Boks to third place in the 2013 World Rugby Under-20 Championship in France.
The 26-year-old, who has also turned out for Bulls in the Currie Cup and Rugby Challenge competitions, told Edinburgh's website he was "honoured" to join the club, adding: "I can't wait to link up with my team-mates.
"I know a lot about the club having spoken to the South African boys in Scotland, and I'm hugely privileged to have been given the chance to represent such an historic side.
"This will be my first time travelling to Scotland, so I can't wait to make the most of this opportunity while sampling a new and exciting rugby culture."
He is the seventh South African at Edinburgh this season, joining Jason Baggott, Nic Groom, Pierre Schoeman, Duhan van der Merwe, Jaco van der Walt and Mike Willemse.
Steenkamp is a former team-mate of Schoeman, van der Merwe and van der Walt, attending the same school as the latter and featuring alongside him for Golden Lions in the 2011 Under-18 Craven Week competition - South Africa's leading high school rugby event.
Edinburgh head coach Richard Cockerill said he was "really happy" to have secured Steenkamp's services, adding: "Ruan is a hard running and abrasive back-row player that plays with a real edge.
"He has all the qualities that you look for in a flanker while his physicality brings a different dynamic to our pack.
“We will obviously be short handed in the back-row in the early stages of the season due to World Cup selection, so to have a player of Ruan’s quality adding depth to the squad is a real coup for the club."
“He has shown in both Super Rugby and the Currie Cup that he has the ability to turn a match in your favour – whether that is through his carrying or tenacity in defence - so we can’t wait to see what he’ll bring to this club."