Mrs May said she would use her meeting to press Britain's call for a credible and transparent investigation into Mr Khashoggi's death in the Saudi consulate in Istanbul in October and urge Mohammed to ensure his country offers full co-operation to the Turkish investigation.
In the video being circulated, Putin and Prince Salman are seen smiling and then smashing a high five before sitting next to each other.
The crown prince's father, King Salman, has publicly backed MBS over recent weeks, and the royal family appears to have publicly accepted that MBS and his succession can not be challenged without seriously disrupting the internal stability of the family and, by extension, the state.
Saudi Arabia has repeatedly denied that Prince Mohammed knew about the murder or played any part in it.
Although U.S. intelligence assessed that Bin Salman likely ordered Khashoggi's killing, President Donald Trump last week decided against imposing sanctions on an important trade ally and Middle East intermediary, saying he was not convinced the crown prince was involved.
Trump, meanwhile, said "we had no discussion".
The first G20 summit to be hosted in South America will wrap up on Saturday. Some 22,000 Argentine police were standing at the ready in case the weekend protests escalated. Much of the capital was placed on lockdown, with public transport suspended. Some painted themselves as national flags.More news: 88 dead, 203 still unaccounted for after Camp Fire contained
President Donald Trump walks to his motorcade vehicle as he arrives on Air Force One.
"No country can solve global problems on its own", he said.
Trudeau is set to hold bilateral meetings with British Prime Minister Theresa May and French President Emmanuel Macron at the meeting in Buenos Aires, Argentina.
The US Senate nonetheless moved this week to end support for the Saudi-led war against rebels in Yemen amid outrage over attacks on civilian sites including a school bus and hospitals.
Putin is under fire, at least most recently, for the Russian seizure of Ukrainian ships and sailors.
The Russian leader also called for securing a nuclear deal with Iran despite the United States withdrawal from it to "prevent a new round of tensions over the Iranian nuclear program".
He warned that Washington's planned withdrawal from the 1987 Intermediate-Range Nuclear Forces Treaty "creates risks of uncontrollable arms race".