The 16-year-old was reported missing on 19 February. Body parts were found at a house in the Barton Hill area of Bristol on 2 March.
Organisers said although the walk "won't bring Becky back" it will help "give this beautiful young girl the send-off she deserves".
Becky's stepbrother Nathan Matthews, 28, has been charged with her murder.
Friends have sold wristbands, T-shirts and balloons to help support her family.
The walk started at 11:00 GMT on The Centre and is making its way to Castle Park where balloons are due to be released in her memory.
Earlier this week, Mr Matthews, from Warmley, Bristol, appeared by video link at Bristol Crown Court.
He has yet to enter a plea to the charge and was told a provisional trial date had been set for October.
His girlfriend, Shauna Hoare, 21, who is charged with perverting the course of justice, also appeared by video link.
Four other defendants have been charged with assisting an offender.
Hundreds of people are taking part in a fundraising walk in Bristol to remember Becky Watts who was killed last month.