The Type-23 frigate left Portsmouth Naval Base to head for the Strait of Hormuz.
Hundreds of people lined the dockside as they waved off HMS Kent.
Britain is at loggerheads with Iran after the regime illegally seized oil tanker Stena Impero.
HMS Kent will join HMS Montrose in the region as the two warships escort every British tanker sailing through the Strait.
She will be taking over duties from Type-45 destroyer HMS Duncan, which is currently in the Gulf.
“Our focus in the Gulf remains firmly one of de-escalating the current tensions”
Commander Andy Brown
HMS Kent’s captain Commander Andy Brown said: “Today’s departure on operations has been achieved through the hard work and support of the ship’s company, their families and the wider defence enterprise.
“Our focus in the Gulf remains firmly one of de-escalating the current tensions. But we are committed to upholding freedom of navigation and reassuring international shipping, which this deployment on operations aims to do.”
The frigate is one of the Duke class vessels, like HMS Montrose, and weighs 4,900 tons.
She is bristling with missiles and torpedoes, can carry a helicopter and also has a 4.5 inch naval gun.
HMS Kent and HMS Montrose will work together to make sure Iran can snatch anymore tankers.
They will be part of an international task force with the US and Israel to protecting shipping in the Strait.
Five tankers and a cargo ship were attacked earlier this year, and Iran claims to have seized three oil tankers – including Stena Impero.
Iran this week also claimed its gunboats chased HMS Duncan, but the video released appears to show the wrong warship.
Iran scooped up the Stena Impero in a tit-for-tat response to the capture of tanker Grace 1 by the Royal Marines near Gibraltar.
Grace 1 is suspected of smuggling illegal oil to Syria, violating EU sanctions.
Both sides are now refusing to back down, with foreign secretary Dominic Raab refusing to negotiate with Iran.
The tanker row tensions come amid the backdrop of the ongoing feud of the so-called nuclear deal, which was scrapped by US President Donald Trump.
Iran is attempting to lockdown the Strait of Hormuz in response to the axing of the nuclear deal which eased sanctions on Iran is response to a promise not to develop books.
Tensions have raged, with the US accusing Iran’s proxy forces of being behind bomb attacks on five tankers in the Gulf.
US President Donald Trump came close to launching a limited strike on Iran after the regime shot down a US drone, but backed down at the 11th hour.
Tehran has broken the binds placed on its enrichment of uranium by so-called nuclear deal with the West – but insist the measures of for power plant fuel not nuclear weapons.
In a statement on HMS Kent’s departure, a spokesman for the Royal Navy said: “Type 23 frigate HMS Kent will relieve HMS Duncan of her duties in the Gulf and is scheduled to work alongside US partners as part of the newly-formed international maritime security construct.
“She will be focused on providing regional maritime security, including counter-terrorism and anti-smuggling activities.
“The deployment aims to strengthen already strong defence ties and demonstrate the UK’s commitment to the peace and prosperity of the region.
“The presence of the ship proves the UK’s global reach with worldwide interests and world-leading defence capability.”