Three will launch its first 5G network this August in London, before extending it to 25 other UK towns and cities before the end of the year.
The telecoms company will kick off with a home broadband service in the capital but is also launching mobile services.
It follows BT's EE and Vodafone in launching next generation 5G services in 2019.
Three, which is owned by Hutchison, said it would not share details about the cost of its services until July.
Chief executive Dave Dyson said: "It's clear that consumers and businesses want more and more data.
"We have worked hard over a long period of time to be able to offer the best end-to-end 5G experience.
EE launched 5G services in six cities in May, while Vodafone will launch one on 3 July.
According to some, 5G will offer download speeds as much as 100 times faster than existing 4G networks.
However, there have been concerns about the pricing and connectivity of such services.
Both have also pulled smartphones made by the Chinese company Huawei from their 5G launches, because of uncertainty about support by Google's Android.
The race for 5G is hotting up - even if Three is arriving with what feels like rather a lukewarm offering.
While EE and Vodafone will both have 5G mobile phones in the hands of customers in a number of cities next month, Three is still only saying that its mobile offering will be rolled out "by the end of 2019".
Its home broadband service is launching in London in August but we still don't know how it will be priced. 5G as a substitute for fixed broadband is an attractive idea - though possibly more useful in remote areas where firms are reluctant to lay fibre connections.
But Three is confident that it has acquired more useful spectrum than its rivals, and when consumers really start to take an interest in 5G next year it will be well-placed to race ahead.
The US has restricted Google and others from using Huawei technology amid concerns it could be used by Beijing for spying - claims which the Chinese firm has denied.
Three said it would announce details of which handsets will be part of its mobile launch in July.
It has suggested its own 5G network will be the fastest in the UK, claiming peak speeds could be more than twice as fast as its rivals.
After London it plans to launch in: Birmingham, Brighton, Bristol, Cardiff, Edinburgh, Glasgow, Leeds, Liverpool, Manchester, Nottingham, Sheffield and Sunderland before the end of the year.
Is 5G safe?
Analysis by BBC Reality Check
Some people have questioned whether there are health risks from 5G, but experts and regulators say there is no evidence of danger.
Similar fears were expressed around earlier mobile internet and wi-fi.
More than 200 scientists appealed to the EU to halt the rollout of 5G, saying that electromagnetic fields may be harmful to humans and the environment, and could increase cancer risks.
But the EU says exposure from 5G will be far below limits set by the International Commission on Non-Ionizing Radiation Protection (ICNIRP).
"There has been no evidence to suggest that electromagnetic waves from mobile phones and networks are bad for your health," says Prof Malcolm Sperrin, Director of the Department of Medical Physics and Clinical Engineering at Oxford University Hospitals NHS Trust.
He says a causal link between mobile phone use and cancer in humans is unproven.
5G technology is new but experts believe it poses no greater risk than earlier mobile systems.