Square has closed its Series D financing round which raised more than US$200 million from investors including Citi Ventures, Rizvi Traverse Management and Starbucks Coffee Company. The company is now valued at US$3.25 billion according to a story in AllThingsD, which referenced “sources familiar with the deal”.

The company will use the new funding to push ahead with its expansion strategy, which includes spreading its service outside the US. It has yet to launch in any international markets. Total funding to date in the start-up is US$341 million.

Among the latest group of investors, Starbucks announced a US$25 million investment in Square in August as part of a wider partnership.

Square says over the past year its business has grown from 150 employees and over US$1 billion in payments processed to today’s figure of over 400 employees and over US$8 billion in payments, an annualised figure.

In a sign of the company's bullishness, COO Keith Rabois predicted that nearly all retailers will be using an iPad or another tablet to process payments instead of a traditional point-of-sale terminal within 18 months.