The outage started shortly before 8:30 a.m. EDT following earlier intermittent problems, CNN reported.
Tumblr had reported the earlier issues in its Twitter feed.
"We're experiencing slow loading or intermittent errors on certain pages and are working quickly to restore performance," a Tumblr post at 7:34 a.m. said.
Shortly before 10 a.m., the blogging site posted again: "Tumblr is experiencing network problems following an issue with one of our uplink providers. We will return to full service shortly."
Launched in 2007, Tumblr hosts more than 77 million blogs and is especially popular with younger Internet users.