Mekosuchinae is an extinct clade of crocodilians from the Cenozoic of Australasia. They represented the dominant group of crocodilians in the region during most of the Cenozoic, first appearing in the fossil record in the Eocene of Australia, and surviving until the arrival of humans: the Late Pleistocene on the Australian continent and during the Holocene in the Pacific islands of Fiji, New Caledonia and Vanuatu.

Mekosuchine crocodiles are a diverse group displaying a great variety of shapes and sizes. Some taxa, like Baru and Paludirex, were large semi-aquatic ambush hunters, though the two genera likely differed significantly in their hunting methods. The medium-sized Australosuchus may have been relatively cold-resistant and taxa like Trilophosuchus and Mekosuchus are renowned for their small size. One of the most distinct mekosuchines was Quinkana, with its altirostral (deep) skull and blade-like serrated teeth.

There is some question around the lifestyle of mekosuchines. Based on skull shape, many taxa are semi-aquatic and most mekosuchines have relatively conservative hip morphology, although other factors might indicate greater terrestrial capabilities. The humeri are straighter than in modern crocodiles, allowing them to perform the so-called "highwalk" more easily. Mekosuchus and Trilophosuchus are commonly regarded as more terrestrial, perhaps similar to dwarf caimans and dwarf crocodiles, while Quinkana displays a skull shape very similar to terrestrial crocodylomorphs like sebecids and planocraniids. In the case of Quinkana, this interpretation is mostly hindered by the near complete lack of postcranial material, with the exception of some isolated hip bones which suggest the presence of a mekosuchine with erect limbs in the Riversleigh World Heritage Area, where Quinkana was found.

Mekosuchines were historically considered to be true crocodiles (of the family Crocodylidae), but modern research favors the idea that they either diverged before the split between gharials and crocodiles or that they are a sister group to Crocodylidae. Some recent studies have even played with the idea that Orientalosuchina, a clade of small crocodilians from the Cretaceous to Paleogene of Asia, might be a part of Mekosuchinae. Regardless of their origins, mekosuchines rapidly diversified between their first appearance during the Eocene and the Oligocene-Miocene boundary, when as many as five different genera of very different morphology all inhabited the freshwater environments and forests of the Riversleigh.

Mekosuchines underwent a decline in post-Miocene Australia, with most genera believed to have gone extinct due to an especially severe period of aridification. While mekosuchines recovered during the Pliocene, the continuous decline of inland freshwater systems and the associated terrestrial biomes gradually lead to the decline of the family. By the Pleistocene only the genera Quinkana and Paludirex still inhabited mainland Australia, alongside the more recent true crocodiles. After the demise of the last mainland mekosuchines, the group survived on Vanuatu and New Caledonia until the Holocene. In both instances the extinction of mekosuchines happens around roughly the same time as the arrival of humans, though it is unclear how much of a part, if any, they played in these events. For the mainland taxa it is argued that their disappearance was entirely related to climate change, with the pattern of extinction matching the disappearance of river basins but not the appearance of humans. Things are more complex for the island forms, with some researchers questioning how much Mekosuchus and humans truly overlapped. If humans played a role in the extinction of the last mekosuchines, it could have been either directly due to overhunting or more indirectly through habitat destruction and invasive species like rats.

Although the family Mekosuchinae was not established until the 1990s, fossil material belonging to members of this clade had been known from the Australian continent for a long time. The first material now recognised as belonging to this group of crocodilians was described in 1886 by English zoologist Charles Walter De Vis. The fossils, discovered in the Darling Downs in Queensland, consisted of skull and postcranial fragments that De Vis dubbed Pallimnarchus pollens. De Vis himself only coined the name "out of convenience", admitting that he was too unfamiliar with the Cenozoic crocodilian fossil record to be certain that his find represented an animal distinct from any other taxa known at the time. Later research has even shown that the material belonged not only to multiple individuals but multiple different genera, with various bones since then having been referred to Paludirex and Quinkana respectively. Regardless of De Vis' caution regarding the taxon, the name Pallimnarchus eventually came to be widely used by other authors.

Despite these early finds, research on genera that we now classify as mekosuchines would grow quiet during the first half of the 20th century, with the lack of published research focused on Australasian crocodilians during the middle of the century leading to a 40-year hiatus. This period would come to an end in 1977 with the publications of Max Hecht, Michael Archer and Ralph Molnar, all of which reported on fossil material collected from cave deposits in northern Queensland. This material encompassed both more fragmentary remains as well as better preserved ones, including a nearly complete rostrum recovered from the Tea Tree Cave. The material was quickly noted for its distinct morphology, bearing some resemblance to terrestrial crocodylomorphs like sebecosuchians and planocraniids. Following the discovery of even more fossil fragments, the taxon was named Quinkana in 1981, though early interpretations linked it to the Paleogene planocraniids rather than the already established Pallimnarchus.

In 1982, just a year later, Molnar published a paper focused on Pallimnarchus, attempting to revise the genus by establishing a lectotype specimen to compensate for the fact that De Vis did not establish a holotype. Around the same time, reports of crocodilian fossil material came out of New Caledonia, these leading to the description of Mekosuchus by French paleontologists Jean-Christophe Balouet and Eric Buffetaut in 1987. Like with Quinkana, the distinct morphology of Mekosuchus initially obscured its relationship to modern crocodilians, with the team placing it in the newly named family Mekosuchidae, which they placed as an early branch of Eusuchia and the sister group to the three extant groups of crocodilians (crocodiles, gharials, alligators and caiman).