The Phlegra Montes are a system of eroded Hesperian–Noachian-aged massifs and knobby terrain in the mid-latitudes of the northern lowlands of Mars, extending northwards from the Elysium Rise towards Vastitas Borealis for nearly 1,400 km (870 mi). The mountain ranges separate the large plains provinces of Utopia Planitia (west) and Amazonis Planitia (east), and were named in the 1970s after a classical albedo feature. The massif terrains are flanked by numerous parallel wrinkle ridges known as the Phlegra Dorsa.

The mountain ranges were first mapped against imagery taken during NASA's Viking program in the 1970s, and the area is thought to have been uplifted due to regional-scale compressive stresses caused by the contemporary formations of the Elysium and Tharsis volcanic provinces. Recent research has unveiled the presence of extensive thrust faulting bounding the massif terrains. Since the 2010s, researchers have proposed the presence of a significant late Amazonian glaciation event along the Martian northern mid-latitudes, citing the presence of lineated valley fills, lobate debris aprons, and concentric crater fills. The presence of ring mold craters imply that significant stores of water ice may continue to persist in these terrains. Features interpreted as eskers have been observed in the southern Phlegra Montes. However, whether this glaciation was localized or of regional scale remains subject to debate in the scientific community.

The Phlegra Montes are a series of sinuous mountain ranges that extends to the north-northeast from the Elysium Rise for nearly 1,400 kilometres (870 mi), dividing the Martian northern lowlands between the Utopia Planitia to the west and the Amazonis Planitia to the east. The southernmost extent of the Phlegra Montes runs up the Elysium Rise and lies due east of Hecates Tholus, the northernmost of the principal volcanic edifices of the Elysium volcanic province. The mountain massifs display heights of up to 3.4 kilometres (11,000 ft), forming one of the most prominent and most extensive mountain ranges on the planet. The ranges' western foothills slope more gradually than those of its more sharply scarped eastern slope. The massifs begin north of Lockyer Crater and near Adams Crater. One portion of the mountain system is isolated in the area north of Adams Crater, with Tyndall crater situated near the center of the range. The chain of massifs ends around 250 kilometres (160 mi) south-southwest of Stokes. The Phlegra Montes take the name of the Phlegra classical albedo feature, which was identified and named by Greek astronomer Eugène Michel Antoniadi in his 1930 publication La Planéte Mars. The International Astronomical Union approved the specific name "Phlegra Montes" in 1973.

West-east-trending valleys score the Phlegra Montes region with a strike that is coincident with the orientation of graben to the west of the mountains, suggesting that regional extensional tectonism affected this region of Mars. Graben along this trend are also present in Galaxias Chaos and into Utopia Planitia. In the Phlegra Montes, some of these graben have been interpreted to host glaciers that have since given way to flow-like landforms called lineated valley fills (LVF). This is not considered a typical way for such landforms to occur on Mars. It has been proposed that the tectonic environment of these valleys was directly controlled by volcanism from the Elysium Rise. The Phlegra Dorsa, a fleet of north-south-trending wrinkle ridges, run largely parallel to the massifs of the Phlegra Montes. These ridges have been morphologically likened by some researchers to sinuous ridges found on lunar maria; they are typically distributed where volcanic flows from the Elysium Rise meet with the older terrains comprising the Phlegra Montes region. Some researchers have found that the formation of these wrinkle ridges has best fit a modeled global stress field involving contributions from the Tharsis Rise, Elysium, and Hecates Tholus (thought to precede Elysium Mons).

The core of the Phlegra Montes is a series of sinuous massifs that are interpreted to be of Hesperian–Noachian age, a greatly degraded remnant of a northern section of the southern Martian highlands terrain. These terrains are pockmarked by steep alcoves and are cross-cut by putatively tectonically formed valleys, which are populated by what have been termed lineated valley fills. In addition to the central massifs, a lobate debris apron (LDA) bounds the margins of the massif. Such debris aprons are better-known for their prevalence around the mesas of fretted terrains across the northern mid-latitudes of the planet. The presence of these features is strongly indicative of a glacial origin. Some researchers have proposed that portions of the Phlegra Montes region were covered by a kilometers-thick glacier in the late Amazonian (within the last few hundreds of millions of years), with the retreat of the glacier responsible for the concentric crater fill and lineated valley fill morphologies. It is possible but not necessary to evoke the presence of a regional ice sheet when explaining these morphologies. There is also evidence of later, superposing lineated valley fills originating from far thinner localized Alpine-like glaciation events that occurred after the retreat of this extremely thick glacial event.

Historically, researchers proposed that the Phlegra Montes ranges may have once been part of a crater rim. Others noted the lack of clear impact structures and have proposed that the ranges were formed through extensional block faulting. Upon an initial review of Mariner 9 imagery in the 1980s, the Phlegra Montes were once proposed to be the only mountain range system on Mars strictly controlled by tectonics, although later examples of extremely linear ridges were later identified within the Claritas Fossae and the Thaumasia Highlands. The notion that the Phlegra Montes ranges might have resulted from compressional activity was first articulated in the 1990s, when researchers identified a potential fold and thrust belt in the Coprates Rise on the Thaumasia Plateau in Tharsis and then analogized the features of that structure to other ridges on the planet (including the Phlegra Montes). The parallel nature of the wrinkle ridges of Phlegra Dorsa was regarded as a critical aspect of this hypothesis. The massifs of the Phlegra Montes were also compared to Amenthes Rupes and Eridania Scopulus.

Norman Sleep, who first proposed a model for plate tectonics on Mars in the 1990s to explain the origin of the northern lowlands, indicated that the Phlegra Montes range might constitute physiographic evidence of a transform-fault plate boundary. Later researchers found that his claim was inconsistent in orientation and fault type with the structural features that were actually present in the range.

More recently, researchers have analyzed the Phlegra Montes as the product of activity on a major Martian thrust fault network. Its asymmetrical profile is characteristic of these putative compressively tectonic structures. Researchers favoring the thrust fault-based interpretation have identified nine major thrust faults bounding the Phlegra Montes massifs, generally to the east. The knobby massif terrains of the Phlegra Montes are typically on the hanging wall of the thrust faults in this region. In some cases, craters (including the large Adams crater in the southern region of the Phlegra Montes) are observed to overprint these putative thrust faults, but not the craters' debris aprons. This is strongly suggestive of ongoing tectonic activity along the identified fault lines.