A jury at Bristol Crown Court took just three hours to find the 28-year-old guilty of murder, kidnap, perverting the course of justice and preventing a lawful burial.
During the harrowing five-week trial, it emerged Matthews suffocated Becky in February at her home in Bristol before dragging her body into the boot of his car and driving to the home he shared with girlfriend Shauna Hoare.
Over the following days - while worried family and friends desperately searched for the pretty teen - Matthews used a saw to dismember Becky’s body before storing the parts in a friend’s shed.
Matthews' girlfriend of six years Shauna Hoare, 21 - who denied all knowledge of the incident - was acquitted of murder but found guilty of manslaughter. Two other men - Donovan Demetrius, 29, and James Ireland, 23 - were acquitted of assisting an offender.
Afterwards, in a statement read outside court, Becky's mother, grandmother and brother paid tribute to her as they said she was a "loyal, loving, smiley, feisty and caring" girl, adding, "cherish your children, as time with them can be too short".
Becky’s stepmother Anjie - Matthews' mother- and father Darren Galsworthy wore blue ribbons in tribute to Becky, and sobbed as the verdict was read out.
For Becky’s school friend Charlie Avery, 16, the verdict came as a relief.
Shop assistant Charlie, who became close friends with Becky at school, says: “Becky was so beautiful, so kind and a genuine friend.
"I miss her so much, she was like a sister. I’m so relieved her killers have been convicted.
"I hate Matthews for taking Becky away. Becky trusted him, and he was supposed to protect her - how could he do this? I can’t believe a family member could harm a loved one like that. It’s so shocking.
“I didn’t go to court but it was really upsetting to read about what Becky went through and I can’t begin to imagine what her family must be going through.
"While nothing will bring Becky back, I hope the verdict brings some comfort to Becky’s family that Matthews will at least pay for what he did.”
To read more of Charlie's story, get your copy of Closer magazine - out now