The High Street store was first opened in 1990 by pop star David Essex, who provided signed pictures to be given as prizes at the celebration party.
Over the quarter century of being open, the store is estimated to have raised more than £1million for the charity.
CLIC Sargent Head of Trading, Jeremy Lune, said: “A huge thank you is owed to everybody who has volunteered, shopped or donated to our Staple Hill shop over the past 20 years.
“Without your support, we would be unable to provide the emotional, practical, financial and clinical support that children and young people so desperately need to help them cope with cancer and get the most out of life."
The anniversary was also marked by the retirement celebration of veteran volunteer Pat Phipps, who stepped down after more than 20 years of service.
Store manager Teresa Tyson said: “CLIC Sargent owes dedicated volunteers like Pat, and her fellow volunteer Pat Smith, who has also been supporting the shop for more than 20 years, a huge debt of gratitude. Their tireless support is inspirational and has helped to ensure the shop’s years of success.
“We are very grateful for the support the local community has shown for us over the last 25 years. The money raised by the shop has helped CLIC Sargent support children and young people from diagnosis onwards, supporting their families as they deal with the impact of cancer and its treatment.”