An official announcement was expected on Monday.
The Yahoo board of directors unanimously approved an all-cash deal worth 1.1 billion dollars, according to the Wall Street Journal.
The two companies were in serious talks last week and the deal was now a foregone conclusion, according to US technology blog AllThings D.
Experts say the acquisition of Tumblr shows Yahoo's desire to attract younger users, especially in the 18-24 age bracket. The Tumblr acquisition would also give Yahoo a fast-growing social networking hub.
Founded in 2007 by entrepreneur David Karp, Tumblr allows users to write blogs and share picture and video content. The site hosts over one hundred million blogs.//DPA