The BT Sport pundit has made a claim after years watching the pair in Scotland and England.
The duo have long attracted comparisons as they both play in the same position for their national team.
Robertson once upon a time was also on the books at Celtic, the team Tierney made his name at, as a youngster and actively supports the club.
Hartson, who has a strong knowledge of Scottish and English football, has now again compared the pair.
Suggesting that the Gunners have done well to bring Tierney to the Emirates for just £25m, he also believes that in one area of the left-back’s game, Tierney is better than last season’s UEFA Champions League winner.
As quoted by The Daily Record, Hartson said: “People talk about Andy Robertson. I would say at this moment in time he has better delivery but Kieran is a better defensive player.
“Kieran gets low and defends well and he can also play as wing-back if needed. I watched Robertson when he was playing for Dundee United when I was covering the game and he was good.
“He went down to Hull and his career fizzled out there but Liverpool took him for £10m and now he is on fire.”
Does Hartson’s assertion have merit?
The BT Sport pundit has probably been a bit hasty with the comparison here. Tierney has yet to even play an English Premier League match never mind prove he can defend at that level consistently.
Robertson has 123 appearances to his name in the top-flight. Last year Liverpool had the best defensive record in the division, in part thanks to the committed Scotland international.
Tierney could well have a similar impact when he gets a run of games with the Gunners. After that the comparison might have more merit.
After his move from Celtic, the 22-year-old still has plenty to prove.