"The Chinese people have lost a good and true comrade," said Xi in his message that was read out at the beginning of the nightly news on the country's main TV channel, AFP reported.
"Comrade Castro will live forever."
The Cuban revolutionary leader, who built a communist state on the doorstep of the United States and for five decades defied US efforts to topple him, died on Friday at the age of 90.
Castro ruled Cuba for five decades, until 2006, when he temporarily ceded power to his brother Raul because he had to undergo surgery. The transfer of power became official in 2008.