No, what3words is not open source.

Creating a new global standard is an ambitious challenge that requires significant resources. We operate as a business to support this work, charging businesses that benefit commercially from using what3words. At the same time, anyone can use what3words for free via our app or online map, and many smaller businesses and organisations also use our technology for free.

Our business model provides the resources needed to constantly improve what3words and develop products that work for all our users, whether they pay us or not. We also put huge effort and resources into driving awareness and use of what3words and believe this is absolutely essential to it becoming a globally applicable solution that anyone, anywhere can rely on and use.