Tony McCluskie, 35, of Bethnal Green, had previously admitted the manslaughter of his sister but denied murder. A jury at the Old Baily found him guilty by a majority of 11-1.
He has been sentenced to life imprisonment and must serve a minimum of 20 years.
Mr McCluskie, a frequant cannabis user, said he did not remember killing the 29-year-old. He told the court he lost control after an argument with his sister and the last thing he recalled was her coming at him with a knife.
Miss McCluskie's headless and limbless body was discovered in Regent's Canal in east London in March. Her arms and legs were pulled from the river a week later, and her head was discovered by a member of the public six months later.
It is believed the actress was killed after the pair had argued about the brother moving out of the Hoxton flat they shared.
The court heard how the argument started when Mr McCluskie left the taps on in the flat, and that his sister had become increasingly infuriated by his drug use prior to the row.
Crispin Aylett QC, prosecuting, said: "Gemma told a friend 'He's permanently stoned. He puts a spliff in his mouth first thing in the morning and doesn't know what he's doing'.
"On Thursday 1 March last year, McCluskie had got up, gone to the bathroom and forgotten the taps were on in the sink. Unsurprisingly, Miss McCluskie was exasperated by this. She had had enough.
"Later, while she was out, her friends heard her arguing on the telephone with her brother about what had happened.
"It is clear that Gemma regarded this incident as the last straw and that she wanted the defendant to move out."
Miss McCluskie died after being hit over the head at least twice and her body was hacked into six pieces.
The following morning, Mr McCluskie tried to cover his tracks by sending his sister a "breezy" text about their mother, who was in hospital at the time. He ended the text with "Love ya xx".
On 2 March, CCTV footage showed him dragging a large heavy bag into the boot of a taxi outside the flat. He then asked the driver to take him to the Regent's Canal.
He was then caught on camera a second time, dragging the bag along the canal.
'Small in stature, huge in personality'
Gemma's father Anthony McCluskie said today: "This has been an extremely traumatic year for myself and my wife Carole, my son Danny and his partner Shantelle, Gemma's extended family and all her many friends.
"To some of her friends Gem was known as 'Gemstar' or 'Li'l Gem' - small in stature, she was huge in personality with a zest for life.
"Gemma was a daughter, a sister, an auntie, a cousin and a niece and is very much loved and greatly missed."
Miss McCluskie played the part of Kerry Skinner in the popular BBC soap in 2001. She went missing after attending the £650m opening of the new Royal London Hospital in Whitechapel, east London.
The defendant took part in extensive searches across east London for the actress following her disappearance.
The investigating officer in the case, acting det chief insp John Nicholson from the Homicide and Serious Crime Command, said: "The investigation into Gemma's death was extremely challenging for all, no more so than for Gemma's family and her many friends.
"No one can imagine quite how horrific it must have been to be told that Gemma had been murdered and that her body had been dismembered, only later to be informed that her brother was suspected as being the person responsible.
"Tony McCluskie tried everything to avoid having the finger of suspicion pointed at him, blatantly lying to police and his family, providing false leads, delaying others from providing crucial information, even sending text messages to his sister's phone whilst she lay dead in the family home; all done in order to cover his tracks and avoid suspicion falling on him.
"The jury saw fit to completely dismiss his pathetic claims that he had amnesia during the period he struggled with, killed, dismembered and ultimately disposed of Gemma's body.
Alison Saunders, CPS London chief crown prosecutor, said: "This was an extremely distressing and violent case. McCluskie used all means possible to divert suspicion away from himself, giving false hope to family and friends.
"We felt that the evidence of brutality showed a deliberate intent to cause death or serious bodily harm to Gemma, which amounts to murder.
"Our thoughts are with those who have been doubly affected by this tragedy."
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