The Electoral Reform Society (ERS) has estimated about 350,000 potential voters are not on the electoral roll.
ERS Cymru director Jessica Blair said a boost in registration and turnout "could make a significant difference" in June.
Voters must be signed up by 22 May to take part in the election.
It is thought students and younger people under the age of 35, people living in the private rented sector, certain Black and Minority Ethnic groups and those from lower income backgrounds are among those who are least likely to be registered, according to data collected last December and adjusted for margin of error.
There are also concerns that numbers of young people registering before they reach voting age - who would now be eligible to vote in June - has dropped in the last few years.
Figures from ERS also claimed the number of 16 to 17-year-olds signing up to register before reaching voting age has fallen by 27% between 2013 to 2016.
The number of so-called attainers dropped from 18,595 to 13,651 in that time. The Vale of Glamorgan saw the eighth worst decline among parliamentary constituencies - 64%.
The figures cover the period when the registration regime changed from a household-based system to a system where individuals sign-up - the switch took place in 2014.
Ms Blair said: "With just a week to go before the deadline to register, these new figures are a wake-up call for everyone in Wales to make sure they are signed up to vote.
"With 350,000 people missing from the register altogether, that's a huge number of citizens in Wales going without a say."
Across the UK more than 930,000 people have applied to register to vote since the 8 June general election was called. More than two million people were registered in Wales at the last general election.
To register online visit the UK government website.
Hundreds of thousands of eligible voters in Wales could go without a say at the general election, campaigners have warned.