Police stopped the pair, aged 61 and 44, as they attempted to carve their names onto a pillar in front of the world-famous monument.
Officers patrolling the attraction – which is frequently the target of vandalism – noticed the two behaving “suspiciously”, before seeing them produce a coin and begin scratching at the pillar.
The two visitors, from Germany and Slovakia, were fined for damaging a building of historical and cultural interest.
They were each slapped with a fine of €450 and a 'daspo', a temporary ban from the area.
Rome mayor Virginia Raggi praised the police for their work, tweeting: “No tolerance for those who deface or damage the monuments of Rome.”
This was just the latest in a long line of incidents of vandalism by tourists at the Trevi Fountain and at other historic sites around Rome, and across Italy.
Police are currently trying to track down a visitor to the Pompeii archaeological site who climbed onto the roof of the baths to take a selfie over the weekend. Visitors are not allowed to touch monuments at the site, which has long suffered problems with vandalism and theft of artefacts.
And last week, Italian police tracked down an Austrian tourist
who was caught on camera snapping three toes off a statue as he posed for a photo at a museum in northern Italy.
The country's monuments are so frequently defaced that heavily-touristed cities such as Rome, Florence and Venice have brought in various local laws aimed at clamping down on the practice, with heavy fines and temporary bans for those breaking the rules.
Cities have also introduced measures intened to prevent unruly or annoying behaviour from visitors, including rules against
everything from “slovenly eating” to pulling wheeled suitcases,.