The tourists were attacked when they were on their way to a temple in the holy town of Pushkar on Monday evening.
The woman said her clothes were torn off and her companion was beaten up when he protested and tried to stop the men from attacking her.
Pushkar is a popular destination among foreign tourists. The town hosts an annual camel fair.
The men who attacked the couple "were consuming liquor", Indian media reports quoted superintendent of police Nitindeep Balaggan as saying.
"The goons attempted to molest a woman tourist and tore her clothes off. They inflicted serious injuries upon her male friend when he tried to intervene," Mr Balaggan told the Hindustan Times.
Indian media reports said the woman was Spanish. The nationality of her companion was not yet clear.
Increasing numbers of rapes and attacks are being reported and highlighted in India, prompting widespread outrage.
Last year, five men were arrested in Kolkata (Calcutta) and charged with kidnapping and repeatedly raping a Japanese student.
In June 2013, a 30-year-old American woman was gang-raped in the northern state of Himachal Pradesh.
A foreign couple has been attacked by a group of "drunk men" in the northern Indian state of Rajasthan, police said.