The outage is the second this year and follows an incident in late February when a glitch took Gmail down for two-hours for users worldwide.
"We're aware of a problem with Gmail affecting a small subset of users. The affected users are having difficulty accessing their accounts. The team is working as quickly as they can to resolve the problem. We know how important email is to our users, so we take issues like this very seriously," Google said.
It is thought Gmail has around 113 million users worldwide, according to ComScore.
See also: Google Docs flaw shares private files