Andrew Smith, from the Campaign Against Arms Trade, which is trying to block United Kingdom arms sales through the courts, said: "We are always being told how much influence the United Kingdom supposedly has over Saudi Arabia".
Asked if May would be raising the issue of Yemen during the visit, her spokesman said it was "not on the agenda".
Mrs May said: "These new partnerships, on defence and security, trade and the economy, education, healthcare, culture and sport, evidence the breadth and depth of the UK's relationship with Saudi Arabia".
In Jordan, May is expected to announce that British military trainers will be sent to help the country's air force fight the Islamic State group.
Ahead of her trip, Mrs May denied the United Kingdom was selling out on its principles - and pledged to raise human rights issues during her meeting with Saudi leaders.
"There is a strong British tradition of standing up for British national interests there".
The prime minister visited Jordan in 2012 in her capacity as Home Secretary.
Her visit takes place amid fears for three prisoners who were arrested as children in 2012 and sentenced to death on charges relating to protests.
"To tackle the threats we face from terrorism and from geopolitical instability, we must meet them at their source", May said, describing Jordan as "on the frontline of multiple regional crises".
May has said she is looking to use the "immense potential for Saudi investment to provide a boost to the British economy" during her trip to the region.
Qatar, a longtime investor in Britain, announced plans last month to invest £5 billion ($6.23 billion/5.8 billion euros) within five years.
In recent weeks the Prime Minister has faced widespread criticism over Britain's arms deals with the Arab state and there have been repeated calls to suspend the sales of weapons amid claims of human rights abuses in Yemen under the Saudi-led coalition bombing campaign.
She was accused of failing to challenge U.S. President Donald Trump and Turkish President Tayyip Erdogan earlier this year.The leader of the opposition Labour Party, Jeremy Corbyn, said May must put human rights and global law at the centre of her talks with the Saudis.
Saudi Arabia launched the offensive against Yemen in March 2015 in an attempt to bring back the former government to power and undermine the Houthi Ansarullah movement.
Fighting between government forces - backed by a Saudi-led coalition of Gulf States - and Houthi rebels in Yemen has left the country on the brink of starvation with thousands dead.
But the kingdom remains determined to prevail in the war next door, where at least 10,000 people have been killed according to United Nations figures, and a year ago exempted active soldiers from cuts to annual leave and bonuses.