Inchtuthil is the name of a large, well-preserved Roman legionary fortress on the banks of the River Tay near Dunkeld in the Scottish county of Perthshire.

It was built in AD 83 as the advance headquarters for the forces of general Gnaeus Julius Agricola who had been steadily fighting his way north from Chester. Positioned at the head of one of the main routes in and out of the Scottish Highlands, it was occupied by Legio XX Valeria Victrix and covered a total area of 21.74ha. Construction of the huge site would have taken more than one season and a temporary camp was built nearby to house and protect the soldiers over the winter. Additional, smaller forts were built further north and south at the mouth of each nearby glen forming what is now referred to as the Glen forts. The relationship of these forts to the Gask Ridge defences concentrated further south is still unclear.

When it was excavated in the 1950s by Sir Ian Richmond a large pit containing more than 750,000 iron nails, weighing ten tonnes, was found. The pit was elaborately concealed and the nails were probably buried by the troops when they left in order to deny them to the local tribes. Unlike other legionary fortresses, Inchtuthil was not later built over and its layout was still largely preserved when Richmond dug it. Its defences consisted of a stone wall with an outside ditch and gatehouses on each side, following the standard Roman plan. It would have been able to accommodate 6,000 troops. Facilities included a hospital that itself covered half a hectare and contained wards for each century, a workshop covering 0.35ha and dozens of barracks buildings, the timber walls of which had a total perimeter of seven miles. A headquarters building and an aedes where the legion's colours and images of the emperor would have been kept have also been identified.

Inchtuthil was only briefly occupied and was evacuated around summer AD 86 and certainly no later than early in AD 87. The reason for this was probably that Legio II Adiutrix had been called to Moesia from its base in Chester to deal with a Dacian invasion in 86 and the XXth Legion was obliged to move back south to take its place. However recent archaeology has cast some doubt on this, indicating that the fortress may have been in use for considerably longer than previously thought.

The 2000 year old iron Inchtuthil nails have been used by atomic scientists to estimate the corrosion effects on barrels of nuclear waste.