The 50-year-old singer had just finished performing his single 'Humble & Kind' when he dropped to his knees before sitting down, reported BBC.
McGraw's wife, singer Faith Hill, later came out to tell the audience he could not continue with the performance.
"He's been super dehydrated. And I apologise, but I made the decision that he cannot come back out on stage," she told fans.
The "It's Your Love" hitmaker had been performing as part of the Country to Country (C2C) music festival in the UK, a three day event which saw him play London on Friday, Glasgow, Scotland on Saturday and Dublin on Sunday.