The Supreme Court struck down a long-established legal clause which meant intercourse between a man and his wife was permissible as long as she was over 15 years of age.
The clause had allowed men to have sex with girls under the legal age of consent as long as they were married.
Indian child protection laws already forbid an adult from having sex with anyone below the age of 18.
But campaigners had argued that the law was of little relevance in the country where, according to UN stats, nearly half (46 per cent) of women aged between 18 and 29 were married before reaching legal adulthood.
On Wednesday, a two-judge bench of India's highest court ruled the age of consent was 18 for "all purposes" after hearing a petition by Independent Thought, a non-profit group that hoped to criminalise sex with underage wives.
The judgement said that girls under 18 would be able to charge their husbands with rape, as long as they filed their complaint within a year of being forced to have sexual relations.
Under the old law, a 17-year-old boy having consensual sex with a girl his age could be charged with statuatory rape, whereas an adult man raping his 15-year-old wife was committing no crime.
Vikram Srivastva, the founder of Independent Thought, said: "This is a landmark judgement that corrects a historical wrong against girls. How could marriage be used as a criterion to discriminate against girls?"
The court stopped short of revoking the law which allows rape within a marriage between two adults. The Delhi high court is currently hearing a challenge to that law, which prevents women from pressing charges for rape committed by their husbands, claiming it could put husbands at risk of "harrassment".
India's government says the practice of child marriage is "an obstacle to nearly every developmental goal: eradicating poverty and hunger, achieving universal primary education, promoting gender equality, and improving women's health."
