The service is available to Three customers travelling to the Republic of Ireland, Australia, Italy, Austria, Hong Kong, Sweden and Denmark.
European authorities have been clamping down on roaming charges.
But Three is the first network to abolish them altogether, albeit in selected countries, where it has sister networks.
Three says customers on a pay monthly contract would be able to use their allowances of free minutes, texts and data with no extra charges for being abroad.
However, those on unlimited plans will be subject to some restrictions.
In a press release, Thomas Malleschitz, marketing director at Three, said: "By abolishing expensive roaming charges in select countries, we are allowing our customers to get even more value from their minutes, texts and data abroad by removing the fear associated with staying in touch while travelling."
Analysts say that other operators of mobile phone networks are likely to follow Three's lead, particularly those who already have a presence in other countries.
Matthew Howett, a senior analyst at consultancy firm Ovum, says the deal from Three is also interesting because it applies to nations outside the European Union, where there has been pressure from authorities to lower roaming charges.
"Reducing roaming charges outside of that bloc will welcomed by many, since prices have remained stubbornly high in some countries," he said.
Three has announced that it has abolished international roaming charges in seven countries.