Russia's regulators said Booking.com had abused its market dominance by preventing hotels from offering lower prices on other services.
Russia's anti-monopoly agency has fined Booking.com a record €14.9 million for alleged market dominance.
The regulator (FAS) said the online accommodation website had abused its "dominant position" and fined the Netherlands-based company 1.3 billion rubles.
Booking.com was accused of preventing Russian hotels and hostels from offering their services at lower prices on other platforms.
The decision came after a complaint by a Russian NGO that defends the rights of small and medium-sized businesses, which is reputed to be close to the government.
"Booking.com abused the dominant position in the Russian market for the provision of information about aggregators' services on distribution facilities," the FAS said in a statement.
"For hotels, this meant that they could not set the prices for their services lower on other sales channels than on Booking.com."
"Such actions restrict competition in the market and harm the interests of hotels," it added.
The FAS said that it had ordered Booking.com to end its "anti-competitive practices" in 2020.
But the regulator said the company had not complied with the order to remove the price parity condition from its contracts with Russian hotels.
In a statement to the Russian TASS news agency, Booking.com said it was "disappointed" by the FAS decision and would appeal the fine.
In recent months, the Russian authorities have stepped up their action against foreign-owned tech companies and social networks.
On Thursday, a court in Moscow also fined Twitter, Facebook, and WhatsApp for violating a law requiring them to store Russian user data in the country.
The heaviest fine of 17 million rubles (€195,000) was imposed on Twitter, while Facebook was fined 15 million rubles (172,000 euros), according to a statement from the Russian internet watchdog, Roskomnadzor.
WhatsApp was also convicted for the first time under the law and was fined 4 million rubles (€45,900).
So far, around 600 companies including Booking.com, Apple, Microsoft and Samsung have complied with the law, Roskomnadzor said.