In a similar thanks cable to the UK Prime Minister, Theresa May, HRH Prince Mohammed bin Salman hailed the establishment of the Strategic Partnership Council between the two countries, noting that the move confirms the determination of the two kingdoms to proceed with enhancing relations between them.
Before his visit, the 32-year-old royal said both countries would be "much safer if you have a strong relationship with Saudi Arabia".
Yemen has been since March 2015 under a brutal aggression by Saudi-led coalition, in a bid to restore power to fugitive former president Abdrabbuh Mansour Hadi.
"If agreed, this shameful deal will be celebrated in the palaces of Riyadh and by the arms companies who will profit from it, but it will mean even greater destruction for the people of Yemen", said Andrew Smith of the Campaign Against Arms Trade.More news: You'll soon be able to upgrade from Windows 10 S for free
Mohammed bin Salman's visit marked by protests against devastating war in Yemen and blockade of Qatar.
His visit to the UK was marred by street protests against arms sales to the kingdom.
Prime Minister May defended the two countries' defence ties on Wednesday, saying all arms sales were strictly regulated.More news: Broadcom sends letter to US Congress, denies national security concerns
On Wednesday, the leader of the opposition, Jeremy Corbyn, criticised May and her government's lack of criticism of Saudi Arabia, arguing it was "colluding" in war crimes by selling arms to Riyadh.
Yemen was a topic of discussion at a private dinner inside May's country residence on Thursday night. After the meeting, however, May awarded bin Salman a family tree showing the descent of the Al Saud dynasty created by Queen Victoria's consul general in Jeddah in 1880.
Andrew Smith of Campaign Against Arms Trade said the United Kingdom had already licensed £4.6bn of arms for Saudi Arabia since the Yemen conflict began in 2015.
The UK and Saudi Arabia announced plans for a £100 million deal yesterday - and it's already has been branded a "national disgrace".More news: Mobile Version Announced with Cross-Platform Play with Nearly All Other Versions