The deal announced on Thursday includes economic aid, support for healthcare, and increased military collaboration on maritime security and drone technology.
Unlike other prime ministers who rushed to Kyiv, Sir Keir took his time to visit, having visited previously as opposition leader.
But after six months in office he came to Ukraine pledging long-term support to help fight what he called Russia’s “illegal and barbaric invasion”.
In Kyiv, the two leaders walked behind an honour guard which carried wreaths in the national colours of the UK and Ukraine.
After soldiers set down the wreaths, Sir Keir and President Zelensky placed their bouquets at the foot of the wall outside St Michael’s Golden-Domed Monastery.
The Kyiv landmark is covered in photos of the dead and has become a place of pilgrimage for grieving families to pay tribute.
As the leaders met for talks inside Kyiv’s Mariinsky palace, multiple explosions and air raid sirens could be heard.
The air raid was “a reminder of the daily attacks and the resolve of the Ukrainian people in the face of it”, Sir Keir said.
Shortly after, when asked by reporters at a press conference what he thought about the ‘hello’ from Russia, Zelensky replied that Ukraine would send its own ‘hello back’.
“We are with you not just today, for this year or the next – but for 100 years – long after this terrible war is over and Ukraine is free and thriving once again,” Sir Keir told the Ukrainian president.
The prime minister also visited an apartment building damaged in a New Year’s Day drone attack, which killed a couple who had been prominent scientists.
The apartment is just down the street from the president’s office.
“They were sending us a message,” the head of Kyiv’s military administration told Sir Keir. “No one is safe”.