This name has never been overly common in the Christian world, and it is considered typically Jewish. It was however borne by an 11th-century Hungarian king.
As a Christian name, Ruth has been in use since the Protestant Reformation. In England it was associated with the archaic word ruth meaning "pity, compassion" (now only commonly seen in the word ruthless). The name became very popular in America following the birth of "Baby" Ruth Cleveland (1891-1904), the daughter of President Grover Cleveland.
In England, it began to be used as a given name after the Protestant Reformation. It was moderately common in the 19th century. It began to rise in popularity again in the late 1980s, probably helped along by characters on the American television shows Friends (1994-2004) and Charmed (1998-2006). It is currently much more common in the United Kingdom, Australia and New Zealand than the United States.
A moon of Saturn bears this name, in honour of the Titan.
This name spiked in popularity in several places around the world in 1954 after Pope Pius declared it to be a Marian year, in honour of the Virgin Mary. A similar declaration in 1987 did not have as marked an effect.
It has been common as a given name in the English-speaking world only since the 18th century. A famous modern bearer is American actress Julia Roberts (1967-).