Sinus Meridiani (Latin Sinus meridiani, "Meridian Bay") is an albedo feature on Mars stretching east-west just south of the planet's equator. It was named by the French astronomer Camille Flammarion in the late 1870s.

In 1979-2001, the vicinity of this feature (with size about 1,600 kilometers (990 mi) and coordinates of the center 7°07′S 4°00′E / 7.12°S 4°E / -7.12; 4) was named Terra Meridiani.

The name Sinus Meridiani was given to a classic albedo feature on Mars by the French astronomer Camille Flammarion in the late 1870s. Prior astronomers, notably the German team of Wilhelm Beer and Johann Heinrich von Mädler and then the Italian Giovanni Schiaparelli, had chosen a particular point on Mars as being the location of its prime meridian when they charted their observations. Accepting suggestions that dark areas on the surface of Mars were seas or oceans, Flammarion named a dark area at that point "Sinus Meridiani," literally "Meridian Bay," when he worked on his compilation and analysis of all prior observations of Mars. In 1958, this name was approved by International Astronomical Union.

Since the 1960s, when flybys and access to orbital spacecraft imagery of Mars began to become commonplace, many relief features were named in addition to previously named albedo features. In 1979, a region of Sinus Meridiani was named Terra Meridiani, literally "Meridian Land." In 2001, boundaries of regional features were redefined, and this name was dropped.

The name Meridiani Planum, literally "Meridian Plain," is used to refer specifically to the landing site of Mars Exploration Rover Opportunity, in the western portion of Sinus Meridiani. This site was chosen by the Mars Exploration Rover team both for its characteristic as a flat and mostly rock-free plain (and hence a safe landing site), and also as a site which showed the spectral signature of the mineral hematite, which is often a sign of deposition in an aqueous environment.

The Opportunity rover found that the soil at Meridiani Planum was very similar to the soil at Gusev crater and Ares Vallis; however, in many places at Sinus Meridiani, the soil was covered with round, hard, gray spherules, dubbed "blueberries." These blueberries were found to be composed almost entirely of the mineral hematite. It was decided that the spectra signal spotted from orbit by 2001 Mars Odyssey was produced by these spherules. Further studies found that the blueberries were concretions formed in the ground by water. Over time, these concretions weathered from what was overlying rock, and then became concentrated on the surface as a lag deposit. The concentration of spherules in bedrock could have produced the observed blueberry covering from the weathering of as little as one meter of rock. Most of the soil consisted of olivine basalt sands that did not come from the local rocks, and is thus speculated to have come from elsewhere.

A Mössbauer spectrum was made of the dust that gathered on Opportunity's capture magnet. The results suggested that the magnetic component of the dust was titanomagnetite, rather than just plain magnetite, as was once thought. Trace amounts of olivine were also detected, which indicated a long arid period on the planet. On the other hand, a small amount of hematite that was present meant that there may have been liquid water for a short time in the early history of the planet. Because the Rock Abrasion Tool (RAT) found it easy to grind into the bedrocks, it is thought that the rocks are much softer than the rocks at Gusev crater.

Few rocks were visible on the surface where Opportunity landed, but bedrock that was exposed in craters was examined by the suite of instruments on the Rover. Bedrock rocks were found to be sedimentary rocks with a high concentration of sulfur in the form of calcium and magnesium sulfates. Some of the sulfates that may be present in bedrocks are kieserite, sulfate anhydrate, bassanite, hexahydrite, epsomite, and gypsum. Salts, such as halite, bischofite, antarcticite, blödite, vanthoffite, or glauberite may also be present.