Mr Zelensky was greeted at the three-day summit by Rishi Sunak, who said the world “stands united” with the Ukrainian people in solidarity against Russia’s invasion.
He said the visit shows the world that Russian brute force and oppression will never triumph over “freedom and sovereignty”.
The Ukrainian leader has been requesting Nato allies to send F-16 jets to help his country resist Russia’s invasion, although he has yet to secure concrete commitments.
Russia, however, warned the West that supplying the powerful jets to Ukraine would incur “colossal risks”.
Russian Deputy Defence Minister Alexander Grushko accused Western countries of "continuing along the path of escalation” over the jets.
On Friday, US president Joe Biden announced his support for training Ukrainian pilots on US-made F-16s, in a possible laying of the ground for eventual transfer of the planes.
Mr Zelensky used the first day of the major international summit to attempt to further galvanise the world’s most powerful democracies against Russia’s 15-month invasion.
Host nation Japan said Zelenskyy’s inclusion stemmed from his “strong wish" to participate in talks with the bloc and other countries that will influence his nation’s defence against Russia.
Mr Zelensky took part in a session with G7 member nations on the war in Ukraine, as well as a sidelines meeting with India’s Narendra Modi, whom he urged to back Ukraine’s peace proposals.
“It is necessary to improve (Ukraine’s) air defence capabilities, including the training of our pilots," he said after meeting Italian Premier Giorgia Meloni, one of a number of leaders he talked to.
The diplomatic efforts coincided with a joint statement by the G7 group condemning Russia’s “brutal war of aggression”.
"Russia’s brutal war of aggression represents a threat to the whole world in breach of fundamental norms, rules and principles of the international community,” the group said.
“We reaffirm our unwavering support for Ukraine for as long as it takes to bring a comprehensive, just and lasting peace," the group said.
The latest sanctions placed on Russia include tighter restrictions on already-sanctioned people and firms involved in the war effort.
Elsewhere on the first day of the summit, G7 leaders issued a joint statement urging China to put pressure on Russia to end the war in Ukraine, while also criticising what they called China’s “economic coercion”.
There is increasing anxiety that Beijing could try to seize the island nation of Taiwan by force, sparking a wider conflict.
China claims the self-governing island as its own and regularly sends ships and warplanes near it.
The G7 said they did not want to harm China and were seeking “constructive and stable relations" with Beijing, “recognizing the importance of engaging candidly with and expressing our concerns directly to China."
They also urged China to pressure Russia to end the war in Ukraine and "support a comprehensive, just and lasting peace."
The leaders began the summit in Hiroshima with a visit to a peace park dedicated to the tens of thousands who died in the world’s first nuclear bomb detonation.
The G7 leaders also discussed efforts to strengthen the global economy and address rising prices around the world on the first day of the three-day summit.
However, Italy’s Premier Giorgia Meloni will skip the last day of the G7 because of floods earlier this week in northern Italy, which claimed at least 14 lives.
The G7 includes Japan, the United States, the UK, France, Germany, Canada and Italy, as well as the European Union.