The "Selfish" author, 35, accompanied the newly engaged couple to a doctor's appointment and to brunch at Nate'n Al of Beverly Hills, reported Us magazine.
The outing marks the first time that Rob, 29, has been spotted out in public with Kim since before he bailed on attending her May 2014 wedding to Kanye West.
As previously reported, Kardashian has also repaired her relationship with former good friend Chyna, 27, since the model started dating her younger brother.
Prior to romancing Chyna, Rob was a recluse and was struggling with depression and weight gain.