Speaking on his first visit to the UK as the technology giant's chief executive, Sundar Pichai said Google plans to build a new 10-storey, 650,000 square foot complex alongside its new complex in Pancras Square.
It will be the first wholly owned and designed Google building outside of the US.
Pichai, in announcing the plans, said: "Here in the UK, it's clear to me that computer science has a great future with the talent, educational institutions, and passion for innovation we see all around us.
"We are committed to the UK and excited to continue our investment in our new King's Cross campus."
Mayor of London Sadiq Khan called the announcement a "vote of confidence" in the city.
"Google's commitment to London is very welcome," he said.
"This is a global, pioneering company at the forefront of new technology - and so London, a pioneering city, and home to many of the world's most talented and innovative people, is a great and natural choice for expansion.
"This is a vote of confidence in our great city - creating high-skilled jobs, supporting growth and demonstrating that London is open to business, new investment and talent from around the globe.
"London is one the world's leading technology hubs and investment into the capital post-Brexit remains robust, so Google's expansion will further strengthen our city's reputation as a global leader in digital technology."
The announcement of Google's planned expansion was also greeted with enthusiasm by Chancellor Philip Hammond.
"This is big vote of confidence in Britain's leading position as a global tech-hub and more evidence that leading firms are choosing to invest here," he said.
"Our technology industry is central to securing future economic growth and this government is committed to ensuring it continues to thrive. It's further proof that Britain is open for business and that we continue to be an outward-looking, world-leading nation."
Pichai, speaking at Google's offices in King's Cross, also admitted the recent US election, which has divided many, had raised "questions of equality" and other "challenges" for the technology giant to face.
Google's search engine was also criticised for promoting allegedly misleading and 'fake news', including a story that suggested President-elect Donald Trump had won the popular vote against Hillary Clinton, despite votes still being counted in some states.
Google apologised, and Mr Pichai reiterated that the firm "does not have all the answers" but will play a role in "addressing these challenges".