Bush, who arrived on Friday, was to speak on democracy and pay homage to Latvia's struggle for independence on Saturday before flying to the Netherlands and Moscow for celebrations commemorating the Allies' defeat of Nazi Germany.
On the last leg of the tour he will visit Georgia.
Hours before greeting Bush at Riga's airport, Latvian President Vaira Vike-Freiberga said Latvia, Estonia and Lithuania were "proud and happy to call America a friend."
"He (Bush) has been sympathetic to us, we who have known oppression, we who have experienced tyranny, we who know what it means to be submitted to the will of others," she said.
She urged the Kremlin to acknowledge the illegality of the Soviet annexation of the Baltic countries, adding that "just once would be enough" to help ease relations with Moscow.
Vike-Freiberga said she expected Bush and Russian President Vladimir Putin to discuss the issue when they meet on Monday in Moscow at the 60th anniversary celebrations of the end of the second world war in Europe.
Bush plans to visit Georgia on
the last leg of his tour
"He has, I understand, talked to President Putin about the situation and the interpretation of history in the past, and seeing as how he is going to Moscow for a historical celebration... I can hardly see how they could bypass the question of history," she said.
Before leaving Washington, Bush told Baltic reporters the United States had always supported the cause of the three nations that suffered under Soviet rule after the Red Army occupied them in 1940.
"Historically, (there is) no question the United States never recognised the form of government imposed upon Latvia, Lithuania and Estonia," Bush said.
He told Lithuanian TV that he would remind Putin about the Soviet occupation when they meet in Moscow.
"The United States never recognised the form of government imposed upon Latvia, Lithuania and Estonia"
Bush was to meet the presidents of the three Baltic countries - some of Nato's newest and smallest members and staunch supporters of the US-led invasion of Iraq.
He was expected to give a speech about democracy and visit the Freedom Monument, dedicated to the country's independence and sovereignty.
Latvia, Lithuania and Estonia joined the European Union a year ago, but there is a widespread feeling in the region that Washington and Nato - not the EU - offer the best protection against their giant eastern neighbour.
Russia maintains that Estonia, Latvia and Lithuania willingly joined the Soviet Union instead of being annexed by Josef Stalin under the 1939 Molotov-Ribbentrop Pact with Nazi Germany.
The whistle-stop trip is Bush's
second to the Baltics
"I do not know what occupation they are talking about. There was no occupation," said Konstantin Sisorin, a Russian-born second world war veteran in his 80s who settled in Lithuania after the war.
Russian troops left the Baltics in 1994, but relations with Russia remain chilly. Moscow routinely accuses Latvia and Estonia of mistreating their large ethnic-Russian minorities, while the Baltics charge Russia with meddling in their affairs.
The 6-7 May visit will be Bush's second trip to the Baltics - he visited Lithuania in 2002.