Seeking more US support for Kyiv's war effort, Zelenskiy, wearing his trademark olive green pants and jumper, earlier met President Joe Biden at the White House.
Zelenskiy presented Biden with a Ukrainian medal offered by a captain of a HIMARS rocket unit and expressed gratitude for the US president's role in helping Ukraine and rallying support.
The United States has sent about $US50 billion ($A75 billion) in assistance to Kyiv as Europe's biggest land conflict since World War II drags on, killing tens of thousands of people, driving millions from their homes and reducing cities to ruins.
But some Republicans, who will take control of the House of Representatives next year, have expressed concerns about the price tag, and European countries have suffered due to energy problems and the hit to the global economy.
They could hold up billions of dollars in war aid starting next month.
"This struggle will define in what world our children and grandchildren will live and then their children and grandchildren," Zelenskiy told a joint session of the US Senate and House of Representatives on Wednesday.
"The world is too interconnected and interdependent to allow someone to stay aside and at the same time to feel safe when such a battle continues."
Members of Congress stood, cheered, applauded and shook Zelenskiy's hand as he entered the chamber, with several wearing the colours of the Ukrainian flag, blue and yellow.
Earlier, Biden and first lady Jill greeted Zelenskiy on the White House lawn before the two leaders spoke at the Oval Office.
On Wednesday, US Secretary of State Antony Blinken announced the US would provide another $US1.85 billion ($A2.76 billion) in military aid for Ukraine including a Patriot air defence system to help it ward off barrages of Russian missiles.
Ukraine has come under repeated Russian missile and drone strikes targeting its energy infrastructure, leaving millions of people without electricity or running water in the dead of a freezing winter.
The Patriot missile is deemed to be one of the most advanced US air defence systems, offering protection against attacking aircraft as well as cruise and ballistic missiles.
Putin was defiant on Wednesday at an end-of-year meeting of top defence chiefs, saying Russian forces were fighting like heroes in Ukraine, would be equipped with modern weapons and would achieve all Moscow's goals.
Putin said there were no financial limits on what the government would provide in terms of equipment and hardware, but the army had to learn from and fix the problems it had experienced in Ukraine.
He gave his backing to a plan by his defence minister to boost the size of the armed forces by more than 30 per cent to 1.5 million combat personnel.
