Jamie Waylett, 20, who plays Vincent Crabbe in the popular films, admitted growing 10 plants at his mother's home.
District Judge Timothy Workman told the young man, who marked his birthday today, that the cannabis cultivation was on a small, but sophisticated scale.
He ordered him to undertake 120 hours of community service during a 15-minute hearing at City of Westminster Magistrates Court.
Mr Workman said: "Mr Waylett I accept that the cultivation of this cannabis was on a small scale, and this was not in any way a commercial venture on your part. Nevertheless you used a sophisticated growing system to do so.
"I give you credit for pleading guilty at the earliest opportunity, your co-operation with police and the fact that you are, until now, a man of good character.
"I propose to deal with this by imposing a sentence of 120 hours of unpaid work."
Waylett and his long-term friend John Innis, 20, were stopped under the Terrorism Act in Lodge Road, St John's Wood, west London, after the actor took a photograph of a police patrol as they drove past.
Innis' black Audi was searched and police discovered a butterfly lock-knife under the driver's seat and eight small bags of herbal cannabis.
When the officers examined the mobile phone on which the shots had been taken, they found images of cannabis plants which Waylett admitted were his.
They then visited his mother's house in Kilburn, north west London, and found 10 cannabis plants growing in a tent in his bedroom.
They found a further three bags of cannabis at a search of Innis' home in Mays Lane, Barnet, north London.
Waylett, of Messina Avenue, Kilburn, admitted production of cannabis at a hearing last week. Innis admitted possession of a knife and having 11 bags of cannabis.
The judge sentenced Innis to six weeks' custody in a young offenders' institution, and fined him £500. He also ordered that all of the drugs and cultivation equipment be destroyed.
Speaking in mitigation for both defendants, solicitor Cheryl Rudden said they have known each other for a long time.
She said both men were "extremely sorry and remorseful" for what had happened.
Mrs Rudden asked the court not to impose a curfew or order Waylett to undertake a specific type of work because it would be a "surplus punishment".
She said: "If I may emphasise that they are both extremely sorry and remorseful for what they have done and certainly would seek to be setting an example. I say that obviously because of the press aspect.
"I am told on Mr Waylett's behalf that there will be no future court appearances. He is feeling the current situation very keenly, as is Mr Innis."
The solicitor added: "Both boys are fit, willing and able to complete the unpaid work. Given the circumstances of both young men it is an appropriate sentence.
"Certainly with a view to future aspirations and perhaps contacts I know the Probation Service is very supportive in that way."
The court heard Waylett had no previous criminal convictions but received a police reprimand and two warnings between 2001 and 2005.
Innis also had no previous convictions but received a police reprimand in 2003. No details of what led to the police action were given by the court.
Waylett left the court flanked by two solicitors after a lengthy private discussion with his legal team. The film star said: "I extend my sincere apologies to the producers, cast and crew and all at Warners and most especially to all Harry Potter fans."
He declined to answer any questions as he prepared to face a large number of photographers waiting outside the court building.
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