Donald Trump chatted with world leaders during a group photo session at the G20 summit, but walked by without appearing to acknowledge Saudi Crown Prince Mohammed bin Salman and Russian President Vladimir Putin.
In contrast, Mr Putin and the crown prince shared a smiley, enthusiastic greeting at the gathering.
The two men beamed widely and clasped hands in something of a cross between a handshake and a high-five.
The crown prince patted Mr Putin’s hand three times for emphasis as the two leaders took their seats next to each other.
The US president cancelled a formal sit-down with Mr Putin at the summit in Argentina over Moscow’s seizure of Ukrainian vessels.
Mr Trump is also not meeting privately in Buenos Aires with the crown prince, who is trying to rebuild his image after the killing of Washington Post columnist and royal family critic Jamal Khashoggi inside the Saudi consulate in Istanbul.