RH, Rh, rH, or rh can stand for:

• Riemann hypothesis, an important unsolved problem in mathematics
• Ryan's Hope, a soap opera
• Rhesus factor, a classification to describe blood types in humans
• Rhodium, a chemical element
• Republic Express Airlines (IATA airline designator)
• Right-handed
• Reinhard Heydrich, a chief of the Reich Security Main Office, Nazi Germany
• Rh (digraph)
• Red Hat, a GNU/Linux operating system
• Restoration Hardware, an American furniture chain of home furnishings, hardware, and outdoor and garden products
• Robin Hood, a heroic outlaw in English folklore
• The RH postal area in the United Kingdom Royal Mail postcode system
• Rolf Harris, Australian entertainer
• Rumbling Hearts, an anime
• Rubeus Hagrid, a character in the Harry Potter series
• Releasing hormone
• Rush hour, the two parts of the day with busy traffic caused by commuting
• Rush Hour (film series)
• Reproductive health
• Rydberg Constant, the Rydberg Constant for Hydrogen
• The Right Honourable
• Radiohead, an English alternative rock, electronica and experimental music band
• Ron Howard, an actor
• Robert Heinlein, an author
• Relative humidity, the amount of water vapor that exists in a gaseous mixture of air and water vapor

The Rh blood group system (including the Rh factor) is one of thirty-five current human blood group systems. It is the most important blood group system after ABO. At present, the Rh blood group system consists of 50 defined blood-group antigens, among which the five antigens D, C, c, E, and e are the most important. The commonly used terms Rh factor, Rh positive and Rh negative refer to the D antigen only. Besides its role in blood transfusion, the Rh blood group system—specifically, the D antigen—is used to determine the risk of hemolytic disease of the newborn (or erythroblastosis fetalis) as prevention is the best approach to the management of this condition. As part of prenatal care, a blood test may be used to find out the blood type of a fetus. If the Rh antigen is lacking, the blood is called Rh-negative. If the antigen is present, it is called Rh-positive. When the mother is Rh-negative and the father is Rh-positive, the fetus can inherit the Rh factor from the father. This makes the fetus Rh-positive too. Problems can arise when the fetus’s blood has the Rh factor and the mother’s blood does not.