The retired Cuban leader compliments Tsipras on what he calls his "brilliant political victory" — an apparent reference to the overwhelming victory of the 'no' campaign in Sunday's Greek referendum on creditor proposals.
Castro praises Greece's historical contributions to philosophy, art, science and politics, as well as its resistance to Axis military powers during World War II.
"Your country, especially your courage in the current moment, awakens admiration among the Latin American and Caribbean peoples of this hemisphere upon seeing how Greece, facing external aggressions, defends its identity and culture," Castro writes.
The letter was dated July 5 and featured on the front page of official newspapers Tuesday.
Meanwhile, Russia is pouring cold water on the prospect that the so-called BRICS nations could help Greece financially at their summit this week.
Russian Economic Development Minister Alexei Ulyukaev told Russian media that the idea of providing any financial help to a non-BRICS member such as Greece is "premature and, it seems to me, not current."
The leaders of the BRICS countries — Brazil, Russia, India, China and South Africa — are due to meet in Russia this week.
Ulyukaev was speaking after a meeting of economic and trade ministers from the BRICS countries.
And the meeting of the 19 eurozone finance ministers has ended and the body's top official, Jeroen Dijsselbloem, is due to issue a statement shortly about the discussions that centered on the Greek crisis. A number of finance ministers left the meeting without speaking to reporters.