"Most services are back online," Yahoo tweeted around 15:30 GMT while attempting to fix the problem that was first flagged more than seven hours earlier.
"This is a technical issue that affected our services. We are sorry for the inconvenience and thank you for your patience as we get everything back up and running," the company added.
According to outage alert service Downdetector, issues have occurred mainly across Europe, parts of the United States and in India.
