Kohli, whose limited-overs leadership stint ended last year, stunned all by relinquishing Test captaincy on Saturday, a day after India lost a three-match Test series to an under-strength South Africa 1-2.
Siraj took to social media on Monday to thank Kohli for “trusting and believing” in his capabilities.
“To my superhero, I cannot be thankful enough for the support and the encouragement that I got from you. You have always been my great brother.
“Thank you for trusting me and believing in me for all these years. For seeing the great at my worst. You will be always be my captain King Kohli @virat.kohli,” Siraj wrote in an Instagram post.
Siraj made his debut in Test, ODI and T20 cricket under Kohli and is also the former skipper’s teammate in IPL franchise Royal Challengers Bangalore.
The 27-year-old pacer from Hyderabad played eight Tests under Kohli, scalping 23 wickets at an average of 27.04.
Both Kohli and Siraj will be seen in action during the ODI series against South Africa starting Wednesday in Paarl.