The move reflects increasing competition in the UK's broadband market dominated by contract or pay-as-you-go services, where a regular hotspot user usually has to pay around 5 pounds for an hour log-on.
Last week, Britain's top fixed-line carrier BT Group said it had teamed up with Wi-Fi crusader FON to allow more than three million UK broadband customers to use hundreds of thousands of hotspots for free.
"The launch of free Wi-Fi in our restaurants in the UK is an exciting move that we believe will further enhance our customers' experience when they visit us," said head of McDonald's Restaurants Ltd, Steve Easterbrook.
"What's more, we hope that this will be a breath of fresh air and give greater choice for Wi-Fi hotspot users who have had little choice but to pay by the month or hour to access the internet on the move."
There are around 11,477 Wi-Fi hotspots in the UK and major service providers include The Cloud, BT, and Deutsche Telecom's T-Mobile.
McDonald's service will account for about 10 percent of the total Wi-Fi hotspots.