Kobo today announced a $50-million investment round.

The financing was led by an unnamed institutional investor, with $13 million raised from existing investors. Indigo maintains its position as Kobo’s main shareholder.

In a statement, Kobo CFO Greg Twinney said, “Kobo will use the new funding to continue its explosive growth internationally.”

The timing of the announcement coincides with the North American launch of RIM’s BlackBerry PlayBook, which comes pre-loaded with the Kobo app. Last week, Kobo announced that in May it will open its first  eBookstores in Germany and Spain, followed by eBookstores for France, Italy, and the Netherlands this summer.

In early March, Kobo announced a Series C investment for an undisclosed amount, and in December 2010, the company raised $16-million from a group of investors that included Indigo, Cheung Kong Holdings, the U.S.-based Borders Group, and REDgroup Retail, which operates Australia’s bankrupt Angus & Robertson and Borders chains.