The acquisition represents the largest deal completed by the search giant since its current CEO, Marissa Mayer, took over in July 2012.
It is understood that under the terms of the acquisition, Tumblr would continue to operate as an independent business, with its founder David Karp remaining in his role as head of the business.
Analysts say that by acquiring Tumblr, Yahoo will gain a larger social media presence and enhance its ability to attract younger audiences in its battle with internet rivals Google and Facebook.