Lokomotiv says the deal includes an option to buy the player for an undisclosed fee.
The deal reunites Joao Mario with forward Eder, his teammate when Portugal won the 2016 European Championship.
Joao Mario says in a statement that "I'm sure we'll get results. I'm looking to the future with inspiration and I'm ready to get out on the field as soon as Saturday."
It's the second time Inter has sent the midfielder out on loan after he spent part of the 2017-18 season with West Ham in the English Premier League.