Antony Blinken arrived by sleeper train from the Polish border, rolling into Kyiv to be greeted at dawn by a VIP delegation.
Ukraine’s wartime railways have become a symbol of resilience: Washington’s weapons approvals for the country have not been such an express service.
The US secretary of state is meeting President Zelensky after Congress finally overcame more than a year of partisan paralysis when it approved £61bn worth of assistance last month.
Ukraine has struggled on the front line while it waited.
On Friday Russia launched a surprise offensive across the border towards Kharkiv while Ukrainian forces continue to fight a grinding battle on the eastern front.
A senior US official said weapons from the newly agreed package have begun to arrive at the front line, including artillery and long-range precision guided missiles.
The official said Blinken would deliver a message of strong reassurance at a difficult moment for Ukrainians, and will have talks on helping the country take back the military initiative from Russia.