A total of 73 members, known as MEPs, will be elected in nine constituencies in England, and one each in Scotland, Wales and Northern Ireland.
Polling stations will be open until 10pm on Thursday - and some voters' local polling stations are a little stranger than others.
Voters have been seen across the country casting their ballots at locations including a caravan, a pub, and even a windmill.
In Hampshire, Petersfield Used Car Centre opened its doors as a polling station for the day.
Some constituents in Hove will cast their votes at West Blatchington Windmill, while a group of Leicestershire electors will head to a farm in Garthorpe to fill in their ballot slip inside a caravan.
Also in Hampshire, some voters will be casting their votes at the White Horse Inn in Priors Dean, also known as the ‘pub with no name’.
In East Lothian, monks Father Mark Caira and Father Leonard Norman, from the Sancta Maria Abbey, cast their votes at Garvald Village Hall.
The results of the election will be announced once all EU nations have voted, with the voting process expected to be completed by 10pm on Sunday.
When Britain leaves the European Union on October 31, the European Parliament will reduce from 751 MEPs to 705, with 27 of the UK's 73 seats being distributed among the remaining member states.
It is uncertain how long British MEPs will sit in the parliament due to Brexit.