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.
The outage comes barely a month after a lengthy Gmail crash left millions of users unable to access their emails. On February 24, Gmail went offline for around two and a half hours, affecting users in the UK and Europe and even as far afield as Australia and India.