The delighted fan invited the Rugby World Cup winning captain over after his side defeated Scotland 30-15 at Murrayfield, Edinburgh.
The video shows Kolisi hurdling the advertisement board as the excited fan waits to receive a unique souvenir.
Someone in the crowd can be heard screaming “yes Siya”.
Kolisi then bends down to sign the interesting pair of pants, while the fan bends over giving a double thumbs up to the crowd.
Once he finished signing the underwear, the Springbok captain gave him a slap on his backside and then embraced the fan.
The crowd cheered him as he ran back onto the pitch.
@Rugbydump posted the video to Twitter on Saturday, writing: ” Meanwhile in Edinburgh.”
The video has now been liked over 19,000 times and had over 4000 retweets from users commenting, finding the interaction hilarious.
@ahh_Wande said: “I loved all of this.
“The Springboks are absolutely everything.”
@Jo43372378 commented: “A legend on and off the field.
“What a Captain to have.”
@bitsof_jay replied: “This is way too funny, first off he stripped to get him to sign his South Africa pants and then the slap has killed me.
“That was a proper slap on the a**e.”
@SineNtombi_ added: “Siya is a man of the people, you guys.
“He gives equal enthusiasm to everyone.”
The loss to South Africa was Scotland’s first of the Autumn tests, having previously defeated Tonga and Australia.
They play their final test game against Japan this Saturday at Murrayfield.