The firm’s valuation has climbed alongside demand for online storage. It was last valued at about $4bn after a 2011 funding round.
Previous investors in Dropbox include Goldman Sachs, Sequoia Capital and Index Ventures.
The six-year-old Silicon Valley startup is expected by many to go public sometime this year. to take advantage of the skyrocketing valuations being applied to fledgling tech companies.
It was reported in November that rival startup Box is also preparing to go public, having been valued at $2bn in a fundraising last month.