Oxford Capital, which started in a garage in Cumnor Hill, made its first investments in 1999.
Since then, it has invested more than £500 million in more than 100 technology companies.
These include Oxford-based start-ups such as machine learning company Latent Logic, Oxford Biotherapeutics, and biotech firm Oxitec, as well as UK-wide companies like Moneybox, Hometree, and Spoke.
Oxford Capital has supported many entrepreneurs and their companies in the Oxford area, including spinouts from Oxford University and firms located in science parks at Oxford, Harwell, Milton, and Begbroke.
The company specialises in helping individual investors back start-up companies via the Enterprise Investment Scheme.
It manages investments in high-growth sectors such as fintech, digital health, cyber security, business software, advanced computing, and e-commerce.
Last week, Oxford Capital was named Investor of the Year at the Great British Entrepreneur Investor Awards.
David Mott, the firm’s founder partner, said: “We love backing the founders of new companies, which makes Oxford the perfect place to be.
“Becoming an entrepreneur is now a career choice for graduates as the cost of starting a business has plummeted over the years – and we see that massively in Oxford.
“Our city now has a buzzing start-up scene with lots of cutting-edge new companies spinning out of the university’s science departments and emerging in the science and business parks in the area.
“We look forward to the next 25 years of backing founders.
“The success of a venture capital firm like ours is down to the trust placed in it from investor clients and the success of the founders that we back.
“It has been an enormous privilege to work with so many wonderful people in Oxford and beyond over the past 25 years.”
Lucia Gunn, director of finance and operations, added: “Oxford is booming – there are exciting new buildings popping up everywhere, often backed by generous philanthropists from around the world who love Oxford.
“There are new labs and thriving networks of entrepreneurs and investors.
“There are brilliant new companies being created – tiny to begin with, but full of ambition, and many of them will grow with the help of Oxford’s unique ecosystem.”
Having hit the 25 year milestone and met thousands of entrepreneurs, Mr Mott offered his advice to people starting a business.
He said two founders are better than one or three and encouraged people to focus on execution, find mentors, and to avoid running out of cash by taking tough decisions to survive.