Bellamy, 31, agreed a season-long loan at the Championship club from Premier League Manchester City so he could "come home" and aid promotion chances.
And Hartson, who played with Bellamy for Wales, thinks it is the right move.
"What else has he got to prove? He was arguably the best player in the Premier League last year," said Hartson.
"I thought his stats proved that, he had a fantastic season for Man City."
Premier League trio Tottenham, Fulham and Everton, along with Scottish Premier League outfit Celtic were also interested in signing Wales captain Bellamy.
But the Wales captain told Tuesday's news conference the chance to spend more time with his family was a prime motivation for dropping out of the top flight to sign for Cardiff.
"My wife and kids live here and I've spent the last six years travelling up and down the road," said Bellamy. "I left home at 15. I've always wanted to come back."
He also said he hoped to help Cardiff win promotion, after they narrowly missed out last season.
"It has been my dream to play my part in taking my hometown club to the Premier League, which would be fantastic for the football club, the city of Cardiff and Wales," Bellamy said on Tuesday.
Bellamy, whose colourful career has taken in eight clubs already and cost nearly £45m in transfer fees, had earlier been given permission to train with Cardiff by Man City boss Roberto Mancini.
The player had already been told he would be left out of City's 25-man Premier League squad due to be finalised by 1 September.
Bellamy says he is "willing to put my neck on the line" to join Cardiff and admitted it is "probably the hardest" football decision he has had to make.
But Hartson insists his former team-mate can still achieve success at Cardiff.
"He's done most things in football - he's played for top clubs, he's played in Europe, he's captained his country every time he plays," said Hartson, who also played with Bellamy at Celtic.
"For Craig, it's a case of going back home and leading his home town club out.
"If he was to take them into the Premier League, what a coup that is for Cardiff.
"It's a great challenge for him to try and get Cardiff up into the Premier League. If he does that, he would have made the right decision."
Source: BBC Sport
Source: BBC Sport